Introduction
Ecosystem productivity in the surface ocean is largely controlled by the
availability of inorganic nutrients. The Redfield ratio describes a constant
molar ratio of C : N : P of 106:16:1 and associates the relative elemental
composition of seawater to that of marine organisms (Redfield, 1958;
Redfield et al., 1963). It provides a widely used basis for the calculation
of elemental fluxes in marine food webs and biogeochemical cycles (Sarmiento
and Gruber, 2006; Sterner and Elser, 2002). On a regional or temporal scale,
however, strong deviations of cellular composition from the Redfield ratio
were reported (Fraga, 2001; Geider and LaRoche, 2002) and related to the
physiological state of cells (Rhee, 1978; Goldman et al., 1979;
Falkowski, 2000; Borchard et al., 2011; Franz et al., 2012a), differences in
growth strategies (Klausmeier et al., 2004; Mills and Arrigo, 2010; Franz et
al., 2012b) and changes in community structure (Sommer et al., 2004; Hauss
et al., 2012). With regard to CO2 uptake and organic matter production,
variations in element stoichiometry of cells have been linked to carbon
overconsumption – a particular increase in carbon assimilation relative to
the uptake of nitrogen and phosphorus (Toggweiler, 1993; Schartau et al.,
2007), when photosynthesis proceeds, while cell division and growth are
hampered due to nutrient limitation (Wood and van Valen, 1990).
A fraction of this “excess carbon” is released from phytoplankton cells in
the form of dissolved organic carbon (DOC). DOC release occurs during all
stages of phytoplankton growth (Fogg, 1983; Mague et al., 1980; Bjørnsen,
1988). In natural communities, the percentage of extracellular release
typically ranges between 10 and 20 % (Baines and Pace, 1991; Nagata, 2000).
Depending on their nutrient status, however, marine phytoplankton cells can
release up to 80 % of primary production as DOC (Sharp, 1977; Mague, 1980;
Fogg, 1983; Bjørnsen, 1988). Thereby, the extent and composition of
freshly produced DOC is affected by various environmental factors, such as
temperature, CO2 concentrations and nutrient supply (Thornton, 2009;
Engel et al., 2011; Borchard and Engel, 2012). Abiotic factors influencing
DOC production concomitantly define its fate in the global carbon cycle. DOC
can either be transferred back to CO2 by microbial degradation and
respiration (Azam, 1983; Ducklow et al., 1986; del Giorgio and Duarte, 2002)
or be transformed into particulate organic carbon (POC), either
through uptake by organisms or by abiotic assembly and coagulation into gel
particles (Alldredge et al., 1993; Chin et al., 1998; Engel et al., 2004a;
Verdugo et al., 2004).
The formation of gel particles thus represents an abiotic pathway of
repartitioning dissolved organic matter (DOM) into particulate organic
matter (POM). To date, mostly two types of gel particles have been described
in seawater: transparent exopolymer particles (TEP), which are rich in carbon
and mainly originate from dissolved polysaccharides, and Coomassie stainable
particles (CSP) that are rich in nitrogen and assumed to form from
proteinaceous compounds (Alldredge et al., 1993; Long and Azam, 1996; Engel,
2009; Cisternas-Novoa et al., 2015). Ubiquitous in the ocean, numerical
abundances of TEP and CSP around 106 L-1 have been reported, with
higher abundances (108 L-1) during phytoplankton blooms (Long and
Azam, 1996; Passow, 2002; Galgani et al., 2014). It has been shown that the
rate of TEP formation during phytoplankton blooms is controlled by the
release rate of dissolved polysaccharides (Engel et al., 2004a). TEP
abundance often increases at times when phytoplankton growth becomes
nutrient limited, either by nitrogen (Corzo et al., 2000; Pedrotti et al.,
2010) or by phosphorus (Borchard and Engel, 2012). In addition to
phytoplankton, often considered the main source of dissolved gel precursors,
bacteria can significantly contribute to the DOM pool and thus to TEP
and CSP formation (Radic et al., 2006; Vadstein et al., 2012). TEP play an
important role in the formation of particle aggregates and therefore can
enhance carbon export fluxes in marine systems (Passow et al., 2001; Engel
et al., 2014). Due to the high carbohydrate content, high abundance of TEP
can increase C : N ratios of suspended and sinking particles in the ocean
(Engel et al., 2002, Schneider et al., 2004; Schartau et al., 2007).
It has been suggested that CSP and TEP are different particles, as their
spatial and temporal occurrence in the ocean can be quite different
(Cisternas-Novoa et al., 2015). Compared to TEP, much less is known for
processes controlling CSP formation. However, it can be assumed that
dissolved precursor concentration and quality are affecting CSP formation in
a similar way that DOC precursors are affecting TEP formation
(Cisternas-Novoa et al., 2014). Thus, CSP formation may be part of the
extracellular cycling of organic nitrogen – i.e. CSP precursors are released
by microorganisms into the dissolved organic nitrogen (DON) pool and
repartitioned into particles by abiotic gel particle formation. Nitrogen is
often considered to be a temporarily limiting element of biomass production
in marine ecosystems, favoring auto- and hetero-trophic nitrogen fixation
(Sarmiento and Gruber, 2006; Deutsch et al., 2007). A labile, extracellular
fraction of organic nitrogen in the form of CSP thus represents a
potentially important source of nutrition. Moreover, extracellular
particulate nitrogen included in CSP may erroneously be attributed to the
cellular nitrogen pool and may hence disguise the real nitrogen cell quota.
Thus, a better knowledge of CSP formation and of the factors controlling CSP
abundance may greatly improve our understanding of nitrogen cycling in
marine ecosystems. So far it is unknown how much CSP contribute to variable
stoichiometry of POM, but we can expect that changes in N : P nutrient
stoichiometry favoring organic nitrogen release also support higher CSP
abundance, potentially increasing the nitrogen fraction in POM.
In this study, we investigated how gel particle formation is affected by
different nitrate and phosphate concentrations during mesocosm bloom
experiments with natural plankton communities collected from surface waters
of the eastern tropical North Atlantic (ETNA), close to Cape Verde. At this
site, surface waters are often depleted in nutrients (Hauss et al., 2013).
Coastal upwelling, N2 fixation or deposition of aeolian dust represent
prevalent pathways of nutrient, particularly inorganic nitrogen, supply to
nutrient-depleted surface waters (Baker et al., 2007; Hansell et al., 2004;
Hauss et al., 2013). On the other hand, anoxic mesoscale eddies have been
described recently in surface waters around Cape Verde, potentially leading
to enhanced nitrogen losses (Karstensen et al., 2015). Thus, pelagic
communities in the euphotic zone of the ETNA are occasionally exposed to
nutrient pulses with different [NO3-] : [PO43-] ratios in
surface waters.
Understanding the impact of changes in nutrient stoichiometry on
phytoplankton communities in the tropical ocean may also help to better
estimate consequences of suboxia on ecosystem productivity and
biogeochemical cycling. Coastal boundary upwelling systems in the ETNA and
eastern tropical North Pacific (ETNP) include some of the largest oxygen
minimum zones (OMZs) in the ocean (< 20–45 µmol O2 kg-1; Gilly et al., 2013). Although they comprise only a small
fraction of the global ocean by volume, they nevertheless play a pivotal
role in controlling the oceans nutrient regimes (Lam and Kuypers, 2011). A
profound loss of the oceanic nitrate stock was suggested to occur in OMZs
(Sarmiento and Gruber, 2006; Codispoti et al., 2001) due to microbial
processes, such as heterotrophic denitrification and anaerobic ammonium
oxidation (anammox; Codispoti and Richards, 1976; Kuypers et al., 2005). As
a consequence, the [NO3-] : [PO43-] stoichiometry of
upwelling water masses above OMZs with strong nitrogen loss often deviates
from the canonical Redfield ratio of 16 (Deutsch et al., 2007). Because
global climate change may lead to an expansion of OMZs, particularly in the
Atlantic and Pacific Ocean (Stramma et al., 2008), future changes in surface
ocean nutrient cycling are to be expected.
Our experiments aimed to identify effects of varied nutrient supply and
stoichiometry on the abundance and size distribution of TEP and CSP, their
dissolved precursors, and the potential impact on carbon and nitrogen
cycling.
Results
Phytoplankton bloom and nutrient development
The development of phytoplankton blooms during the mesocosm experiments and
the build-up of particulate matter are described in more detail in Meyer et al. (2015) and are summarized here only briefly.
Before nutrient addition (day 0), Chl a concentration was on average 0.38 ± 0.09 µg L-1 in all mesocosms of Varied P and hence higher than
at the start of Varied N with 0.18 ± 0.05 µg L-1 (Table 1). As long
as nutrients were replete, bloom development was similar in all mesocosms
within each experiment (Fig. 1a–f). However, during Varied P most mesocosms reached
maximum Chl a concentrations, i.e. bloom peak, on day 5 and thus 1 day
earlier than during Varied N (Fig. 1a, b). Maximum Chl a concentration ranged
between 2.1 and 3.3 µg L-1 during Varied P and between 2 and 10 µg L-1
during Varied N. Hence, during Varied N higher concentrations of Chl a were
determined as well as a higher variability among mesocosms. During both
experiments, MDs of Chl a concentration in the different
mesocosms were correlated to the concentration of the initial nutrient
varied, i.e. PO43- during Varied P and NO3- during Varied N,
although the response was much stronger during Varied N (Table 2). The phytoplankton
biomass composition was dominated by diatoms (data not shown). Diazotrophic
bacteria of the genus Trichodesmium were more present in the initial waters of Varied P, while
proteobacterial diazotrophs were more abundant in Varied N (Meyer et al., 2015).
Bloom development during two series of mesocosm
experiments, one with varied supply of PO43- (Varied P; a, c, e; n= 16)
and one with NO3- (Varied N; b, d, f; n= 16). Shaded areas indicate the
range (min–max) of data observed during both treatments for Chl a
concentration (a, b), bacterial abundance (c, d) and particulate organic
carbon (POC) concentration (e, f).
Bacterial abundance was not determined before nutrient addition, but data
from day 2 showed higher abundance in mesocosms of Varied N with 8.37 × 105 ± 9.80 × 104
compared to 5.26 × 105 ± 5.48 × 104 mL-1 for Varied P. During the first 4 days of both experiments,
cell numbers remained relatively stable or even decreased slightly (Fig. 1c, d). After day 5, cell numbers increased in all mesocosms and strongly
differed between treatments. During Varied P, different PO43- addition
could not significantly explain differences in bacterial abundance (Table 2). Instead, highest abundances in the bloom phase were reached in the
“centerpoint” treatment (12.0N / 0.75P), with a maximum value of 2.4 × 106 ± 7.1 × 105 mL-1
on day 6 compared to 2.3 × 106 ± 7.1 × 105, 1.5 × 106 ± 3.0 × 105 and
1.7 × 106 ± 8.4 × 105 mL-1 in treatments 12.0N / 0.25P, 12.0N / 1.25P and
12.0N / 1.75P, respectively. During Varied N, bacterial abundances were positively
influenced by NO3- input (Table 2). At the end of the experiment
(day 8), 1.6 × 106 ± 4.7 × 105 and 2.3 × 106 ± 5.4 × 105 mL-1
cells were observed in the high-NO3- treatments 20.0N / 0.75P and 12.0 / 0.75P, respectively, compared to
8.1 × 105 ± 1.4 × 105 and 1.0 × 106 ± 1.5 × 105 mL-1 in 2.0N / 0.75P and 4.0N / 0.75P, respectively.
Mean deviations (MDs) from the average development, averaged for
each mesocosm for the full experimental period (day 1–8) and Pearson
coefficients for correlations of MD versus initial PO43- (Varied P; µmol L-1) and
NO3- (Varied N; µmol L-1) concentrations; bold
numbers indicate significant correlation (p < 0.05).
Varied P
Chl a
Bact.
POC
PN
DOC
DON
TEP
CSP
Varied N
Chl a
Bact.
POC
PN
DOC
DON
TEP
CSP
treatment
µg L-1
105 mL-1
µM
µM
µM
µM
106 L-1
106 L-1
treatment
µg L-1
105 mL-1
µM
µM
µM
µM
106 L-1
106 L-1
12.0N / 0.75P
-0.21
2.84
2.45
0.35
3.81
-0.27
4.42
2.07
12.0N / 0.75P
0.15
-1.70
4.24
0.61
3.32
1.13
-6.47
-4.23
12.0N / 0.75P
0.03
-0.65
1.93
0.14
-1.39
0.54
21.16
3.32
12.0N / 0.75P
0.23
1.12
4.87
0.56
2.70
-0.63
1.78
14.98
12.0N / 0.75P
0.06
-0.69
3.35
0.71
-0.74
0.00
16.00
4.73
12.0N / 0.75P
0.64
1.10
11.32
1.34
-0.67
-0.88
5.09
12.60
12.0N / 0.75P
0.06
3.61
-0.10
0.29
-4.41
0.19
0.53
-5.63
–
–
–
–
–
–
–
–
17.65N / 1.10P
0.54
4.14
4.07
0.83
3.15
-0.38
7.77
5.48
17.65N / 1.10P
0.76
4.24
2.34
0.75
-1.35
-0.52
-7.40
8.06
12.0N / 0.25P
-0.28
-1.44
-8.30
-1.14
-4.86
0.49
-19.47
3.39
2.00N / 0.75P
-1.01
-1.58
-4.51
-1.14
-2.27
-0.48
-15.10
-18.80
12.0N / 0.25P
-0.26
-0.46
-6.51
-0.99
-4.39
0.56
-17.77
-6.21
2.00N / 0.75P
-1.04
-1.89
-14.10
-2.12
-9.27
3.42
-21.78
-19.77
12.0N / 0.25P
-0.30
-1.09
-2.00
-0.67
-0.10
0.13
-17.16
-4.46
2.00N / 0.75P
-1.09
-2.20
-8.85
-1.38
-18.84
-2.18
-15.68
-14.42
6.35N / 1.10P
-0.29
-0.79
-0.47
-0.35
-2.48
-0.51
5.83
-4.87
4.00N / 0.75P
-0.62
-0.52
-5.46
-1.05
-3.88
-0.65
1.07
-9.07
12.0N / 1.25P
0.03
0.28
4.86
0.58
2.84
-0.41
0.59
0.15
4.00N / 0.75P
-0.88
-0.88
-10.83
-1.58
-4.78
0.25
-21.95
-9.19
12.0N / 1.25P
0.16
-0.73
0.39
0.10
5.46
0.24
10.52
-2.35
4.00N / 0.75P
-0.22
-0.79
-6.56
-1.54
-2.37
-1.60
3.12
-9.56
12.0N / 1.25P
0.38
-1.32
2.71
0.19
4.69
0.06
6.48
-6.93
6.00N / 1.03P
-0.65
-2.14
-12.28
-1.73
-0.08
-1.34
-23.60
-8.95
12.0N / 1.75P
0.13
-2.16
-1.61
-0.40
-2.89
0.04
3.00
4.16
6.35N / 0.40P
-0.49
1.26
0.10
-0.25
0.53
0.11
-5.21
-0.84
12.0N / 1.75P
-0.27
-0.83
-4.35
-0.19
4.07
-0.12
-15.74
4.07
17.65N / 0.40P
0.91
-0.79
7.45
1.17
6.22
-0.30
15.35
16.30
12.0N / 1.75P
0.23
-0.70
1.66
0.26
-2.75
0.51
-6.15
3.07
20.0N / 0.75P
0.94
1.62
10.52
2.08
7.68
1.80
39.38
19.81
20.0N / 0.75P
1.05
1.85
7.29
1.51
1.51
-0.12
17.43
-1.03
20.0N / 0.75P
1.31
1.30
14.47
2.76
4.16
0.78
33.98
24.11
r(PO43)=
0.45
-0.10
0.37
0.41
0.39
-0.24
0.25
0.31
r (NO3-) =
0.97
0.70
0.87
0.94
0.71
0.18
0.80
0.86
Initial concentrations of POC were 13.6 ± 3.8 and
11.9 ± 1.9 µmol L-1 during Varied P and Varied N, respectively (Table 1).
During both experiments, concentrations increased steadily until day 6 and
remained relatively stable thereafter (Fig. 1e, f). POC concentrations
during Varied P were up to 73 µmol L-1 (17.65N / 1.10P), but not related to
the initial PO43- addition. In contrast, build-up of POC was more
pronounced during Varied N, with values up to 102 ± 18 µmol L-1
determined in treatments with the highest initial NO3- supply
(20.0N / 0.75P), and indicated a clear correlation to the initial
NO3- treatment (Table 2).
Along with plankton growth, inorganic nutrient concentrations declined (Fig. 2).
During Varied P, PO43- was exhausted on day 5 in the treatments with
the lowest initial PO43- supply and the highest initial
[NO3-] : [PO43-] ratio of 74, i.e. 12.0N / 0.25P. In all
other treatments, PO43- depletion was reached later during the
experiment, except for the highest PO43- treatment (12.0N / 1.75P), in which PO43- remained
> 0.3 µmol L-1 until the last experimental day. During the same
experiment, NO3- concentrations fell below the detection limit
of 0.03 µmol L-1 in some of the mesocosms after day 5 but were
not depleted in 12.0N / 0.25P. During Varied N, NO3- was exhausted on day 5
in the low-N-supply mesocosms (2.00N / 0.75P and 4.00N / 0.75P). On day 6,
NO3- was still available in treatments with an initial nitrate
supply > 12 µmol L-1, and on day 8 NO3- was
only available in 17.65N / 0.40P, the mesocosms with the highest
[NO3-] : [PO43-] ratio of 84. After the bloom,
PO43- was below the detection limit in 9 out of 16 mesocosms
with [NO3-] : [PO43-] ratios > 10.
Nutrient concentrations during two series of mesocosm
experiments with varied supply of PO43- (Varied P; a, c; n= 16) or
NO3- (Varied N; b, d; n= 16), respectively. For treatments with identical
nutrient supply, average values are given ±1 standard deviation
(error bars). See Table 1 for explanation of symbols.
Dissolved organic carbon (DOC) and nitrogen (DON)
Averaged for all mesocosms, initial (day 1) DOC concentration was very
similar for Varied P (95 ± 5 µmol C L-1) and Varied N (96 ± 4 µmol C L-1; Table 1). Throughout both experiments, DOC
concentrations increased steadily after day 2, except for day 5, when a
slight decrease was observed in most mesocosms (Fig. 3a, b). For Varied P,
accumulation of DOC with respect to initial concentration (day 1) (ΔDOC) was observed, ranging from 18.8 ± 6.7 µmol L-1
(12.0N / 0.25P) to 44.0 ± 12.0 µmol L-1 (12.0N / 0.75P). During
Varied N, ΔDOC increased also in the course of the experiment, with highest
values observed at the end of the experiment, ranging from 12.1 ± 1.1 µmol L-1 DOC in the treatment with the lowest nitrate supply
(2.0N / 0.75P) to 74.4 ± 16.6 µmol L-1 in 12.0N / 0.75P 75,
the same treatment that yielded highest ΔDOC during Varied P. MDs of DOC were
not significantly correlated to the initial PO43- supply during
Varied P but rather to the initial NO3- supply during Varied N (p < 0.005), indicating a
general dependence of DOC accumulation on NO3- stocks (Table 2).
On day 1, DON concentration (day 1) was 8.8 ± 1.1 and 11 ± 1.5 µmol L-1 for mesocosms of Varied P and Varied N, respectively. In both
experiments, DON concentration decreased after nutrient addition (Fig. 3c,
d). During Varied P, ΔDON was negative in some of the mesocosms until the
Chl a maximum on day 5. Values increased slowly between days 6 and 7 before a
clear increase was determined for all mesocosms on day 8 with ΔDON
accumulation ranging between 1.9 and 5.9 µmol L-1 (Fig. 3c).
During Varied N, a clear accumulation of DON was not observed, yielding values of
ΔDON of -6.0 to 4.8 µmol L-1 at the end of the
experiment. On day 8 of both experiments, highest and lowest ΔDON
were determined in the treatments with the highest and lowest initial
NO3- supply at identical PO43- supply, respectively. A
significant correlation between the initial PO43- or
NO3- supply and DON accumulation, however, was not determined
(Table 2).
Changes in dissolved organic carbon (δDOC; a, b) and dissolved organic nitrogen (δDON; c, d)
concentration during Varied P (a, c) and Varied N (b, d). Values are given as difference to
day 1. For treatments with identical nutrient supply, average values are
given ±1 standard deviation (error bars). The dashed line visualizes
the zero value; symbols as in Table 1.
Both increasing DOC and decreasing DON concentrations resulted in a rise of
molar [DOC] : [DON] ratios until the bloom peak during both experiments (data
not shown). During Varied P [DOC] : [DON] ratios were initially 10.1 ± 0.92,
averaged for all mesocosms and ranged between 7.7 and 31 throughout the
experiment, with highest values being observed just before the bloom peak.
During Varied N, [DOC] : [DON] ratios started at 9.1 ± 1.1 and ranged between 6.8
and 34 throughout the experiment, with highest values also observed shortly
before the bloom peak on day 6.
Gel particle abundance
Averaged for all mesocosms, initial (day 1) TEP numerical abundance was 0.97 ± 0.64 × 107 L-1 for Varied P and steadily increased to highest
values between 5.9 × 107 and 1.5 × 108 L-1 until the end of
the study (Fig. 4a). TEP total area behaved similar to TEP numerical
abundance; values increased from an initial 4.46 ± 2.36 × 107 to values between
3.9 × 108 and 7.9 × 108 µm2 L-1 on day 8 (data not shown).
Variation of initial NO3- concentrations during Varied N induced clearly
stronger responses in TEP formation than variation in initial
PO43- concentration (Fig. 4b). From an averaged 1.07 ± 0.34 × 107 L-1, TEP abundance increased until day 8 to
values of 1.1 × 108–2.8 × 108 L-1. While initial numbers were in a
comparable range for both experiments, the maximum TEP abundances (day 8)
during Varied N were about twice as high as during Varied P. The same holds for TEP total
area: initial averaged values were only slightly higher (5.04 ± 1.43 × 107 µm2 L-1) than initial values during
Varied P, but highest values more than doubled on day 8 during Varied N, yielding 9.6 × 108–1.6 × 109 µm2 L-1 (data not
shown).
During both experiments, TEP numbers and total area increased similarly in
all treatments until the Chl a maximum. From day 6 onwards, however, distinct
differences emerged between treatments, particularly during Varied N. Here, TEP
abundance was significantly higher in the highest NO3- treatment
(20.0N / 0.75P) compared to treatments amended with lower nitrate supply
(2.0N / 0.75P; p < 0.001, 4.0N / 0.75P; p < 0.005, 6.0N / 1.03P; p < 0.05). On day 7, TEP
numbers in 20.0N / 0.75P reached their maximum and were significantly higher
than in all other treatments (p < 0.001), where TEP numbers continued to
increase on day 8. Like TEP numbers, TEP total area was also significantly
larger in the highest NO3- treatment (20.0N / 0.75P) compared to
2.0N / 0.75P and 6.0N / 1.03P (p < 0.01), showing a clear stimulation of TEP
formation at higher nitrate levels.
Temporal changes in the total numerical abundance of
transparent exopolymer particles (TEP; a, b) and of Coomassie stainable
particles (CSP; c, d) during Varied P (a, c) and Varied N (b, d). For treatments with
identical nutrient supply, average values are given ±1 standard
deviation (error bars). The dashed line visualizes the zero value; symbols
as in Table 1.
For Varied P, initial PO43- concentration had on average no significant
effect on MD of TEP abundance (Table 2). In contrast, a significant positive
relationship between MD of TEP abundance and initial NO3- supply
was determined during Varied N (p < 0.001). This relationship, however, reversed when
MDs
of Chl a-normalized TEP concentration were considered, indicating that a
relatively higher fraction of newly fixed organic carbon was partitioned
into TEP at lower nitrate supply on a cellular level (p < 0.001; data
not shown).
Similar to TEP, CSP abundance and total area increased steadily over time
during both mesocosm experiments, although CSP were generally less abundant
than TEP (Fig. 4c, d). From an initial mean value of 1.06 ± 0.61 × 106 L-1 during
Varied P, CSP numerical abundance increased to 4.2 × 106 to 1.0 × 107 L-1 on day 8. Highest CSP abundance was determined
in the treatment with the highest nitrate supply (17.65N / 1.10P), where CSP
total area of initially 1.5 ± 0.5 × 107 µm2 L-1 increased to
4.5 × 107–1.2 × 108 µm2 L-1 on day 8. Similar to TEP, a much stronger
increase in CSP abundance was observed during Varied N. Here, CSP numbers increased
from an initial average of 1.63 ± 0.48 × 106 L-1 to highest
values of 1.4 × 107–2.8 × 108 L-1 on day 7 (Fig. 5d) – more
than double the amount observed during Varied P. Again, highest CSP abundances were
determined in replicate treatments of highest NO3- supply
(20.0N / 0.75P), yielding 2.7 ± 0.1 × 107 L-1.
Analysis of variance for data obtained on day 7 revealed significantly
higher CSP abundances in 20.0N / 0.75P compared to 2.0N / 0.75P (p < 0.001),
4.0N / 0.75P (p < 0.001) and 6.35N / 0.40P (p < 0.05), indicating a stimulation of CSP
formation at elevated initial NO3- concentrations. This is in
accordance with a highly significant correlation of MD of CSP abundance and
initial NO3- concentrations (p < 0.001, Table 2). Findings
for CSP numbers are reflected in CSP total area: highest values were also
observed for the high-NO3- treatment (20.0N / 0.75P;
213 ± 21 × 106 µm2 L-1), with values
significantly larger than in 2.0N / 0.75P (p < 0.005), 4.0N / 0.75P (p < 0.001),
6.35N / 0.40P (p < 0.001), 17.65N / 1.10P (p < 0.05) and 12.0N / 0.75P (p < 0.005; data not
shown). In contrast to TEP abundance, CSP number declined in most treatments
on day 8 of Varied N (except for 12.0N / 0.75P; only MK 1, 6.35N / 0.40P and 4.0N / 0.75P;
only MK 11).
Size–frequency distribution of gel particles.
Calculation of the size–frequency distribution slope exemplified for TEP,
averaged for all mesocosms on day 1 (circles) and 8 (triangles) during
Varied P (a) and Varied N (b), respectively, and for CSP on day 2 (circles) and 8 (triangles)
during Varied P (c) and Varied N (d). Linear regression of log[dN/d(dp)] versus log[dp]
was fitted to the particles in the size range of 1.05–14.14 µm ESD
(solid symbols).
Gel particle size distributions
At the beginning of the study, median values for TEP equivalent spherical
diameter (ESD) were almost identical for Varied P and Varied N, yielding 1.78 ± 0.12
and 1.79 ± 0.08 µm ESD, respectively. Except for days 6 and 8,
median size of TEP was steadily increasing over time in Varied P, with largest
particles occurring in 6.35N / 1.10P, 12.0N / 1.75P and 12.0N / 1.25P on day 7
(2.28–2.30 µm ESD). On day 8, median TEP size was slightly smaller
again and similar in all treatments ranging between 1.80 and 2.26 µm
ESD. During Varied N, the size of TEP remained relatively constant between days 1 and 4 and
then increased until the Chl a maximum. After the bloom peak, median TEP size
further increased until day 6, yielding values between 2.5 and 1.9 µm
ESD at the end of the experiment.
Spectral slopes describe the size–frequency distribution of particles with
more negative values indicating relatively more small particles (Fig. 5) and
mirrored changes in the median ESD of both types of gel particles during
both experiments. Changes in size–frequency distribution of TEP were
observed for Varied P and Varied N, with slope values (δ) becoming significantly
smaller during the first half of both experiments (p < 0.001; multiple
comparison, Holm–Šidák; Fig. 6). Average slopes on day 1 were very similar
for Varied P and Varied N, yielding δ=-1.81 ± 0.12 and δ=-1.81 ± 0.11, respectively. Slopes increased to average -1.44 ± 0.06
(Varied P) and -1.38 ± 0.06 (Varied N) on day 8 of both experiments, suggesting a
relative shift from smaller to larger TEP (Fig. 6a, b).
Changes in the slope (δ) of the size–frequency
distribution of TEP (a, b) and CSP (c, d) during the mesocosm blooms. The
grey lines indicate the mean value of all mesocosms on the respective day;
symbols as in Table 1.
Slightly smaller than TEP size, median CSP size was on average 1.37 ± 0.06 µm ESD at the beginning of Varied P, and increased to values between 1.13 and
1.78 µm ESD until the end of the experiment (Fig. 6c). During Varied N, median
CSP size increased between day 1 (1.36 ± 0.09 µm ESD) and day 4
(1.34–1.85 µm ESD). In contrast to median TEP size, median CSP
size decreased towards the end of the experiments and ranged between 1.18 and
1.71 µm ESD on day 8. During Varied P, a large variability in δ values
was observed for CSP size distribution on day 1. To estimate changes in size
distribution during this experiment, data evaluation of CSP slopes was
started on day 2 (Fig. 6c), when CSP size distribution was more similar
between mesocosms. Like for TEP, development of CSP spectral slopes during
this study mirrored the change in median ESD size of particles. Averaged for
all mesocosms, δ=-1.40 ± 0.14 was obtained on day 2 of
Varied P, increasing steadily to -1.24 ± 0.23 until day 8. The size–frequency
distribution of CSP during Varied P was not affected by the initial nutrient supply.
For Varied N, initial slopes also scattered on day 1, however not as strongly as for
initial values for Varied P. Initial averaged slopes for all mesocosms were -1.64 ± 0.28 (Fig. 6d). During days 2–4, the overall development shows a
relative increase in the slope of the size distribution during the onset of
the bloom. Highest values of δ=-0.84 coincided with the largest
median ESD of CSP on day 4. At the time of the Chl a maximum, slopes became
more negative, revealing higher abundance of relatively small particles.
Multiple comparison (Holm–Šidák) tests revealed significantly larger slopes
for days 2 to 4, compared to days 1, 6, 7 and 8 (p < 0.010). The increase in
abundance of smaller CSP continued during the bloom decay and was most
pronounced in 2.0N / 0.75P and 4.0N / 0.75P, the treatments with the lowest
initial NO3- supply.
Differences between two mesocosm experiments – a case of
treatment effects?
Although the development of gel particle abundance was rather similar for
TEP and CSP during both experiments, particularly until the bloom peak,
abundance of gel particles was clearly higher during the second mesocosm
experiment, Varied N, compared to Varied P (Fig. 4). Moreover, during Varied N, CSP increased
relatively more than TEP and showed a unique change in size distributions
during bloom development not observed during Varied P and different from TEP.
In order to identify differences between the two series of mesocosm
experiments, gel particle abundance was compared to bloom development, which
also differed between the experiments.
Relationships between organic components during
Varied P and Varied N. Solid symbols: data obtained during Varied P; open symbols: data obtained
during Varied N. Linear regressions with Chl a include data of samplings 1, 2 and 5
for Varied P and 1–6 for Varied N. Linear regressions with POC and PN include data of all
samplings. Information on regression statistics is given in Table 2.
During both experiments, gel particle dynamics were tightly coupled to the
production of organic matter during bloom development (Fig. 7, Table 3).
Numerical abundances of TEP and CSP were directly related to Chl a
concentration until the bloom peak (Fig. 7a, d). Thereby, the increase in
gel particle abundance with Chl a concentration was different for TEP and CSP
during Varied P as well as during Varied N. While TEP abundance increased slightly faster
with Chl a concentration during Varied P, the increase in CSP abundance with Chl a
concentration was twice as strong during Varied N compared to during Varied P (Table 3). After the
Chl a maxima, gel particle formation continued while Chl a concentrations
declined, leading to higher [gel particles] : [Chl a] ratios towards the end of
the experiments (Table 4). Partly decoupled from Chl a concentration, gel
particles remained tightly coupled to POC and PN dynamics throughout both
experiments (Fig. 7b, c, e, f). Thereby, a similar coupling was observed
between TEP and POC or PN concentration during both experiments, while CSP
abundance increased more strongly with POC and PN concentration during
Varied N (Table 3). The carbon content of TEP (TEP-C) averaged for all mesocosms on
day 1 was 0.61 ± 0.29 and 0.72 ± 0.38 µmol L-1 for Varied P and Varied N, respectively. During both experiments, TEP-C
steadily increased along with the general abundance of TEP. Maximum TEP-C
during Varied P was reached on day 8, with values of 12.6–34.9 µmol L-1
representing a share of 31–41 mol % POC, or 8.4–17.6 mol % TOC (Table 4). During Varied N, final TEP-C concentration contributed with an even higher
proportion to the organic carbon pool, equivalent to 22.8–84 mol % POC or
12–29 mol % of TOC. Molar ratios of [TEP-C] : [PN] were initially below 1
and increased to an average of 2.2–3.6 during Varied P and to 1.8–6.9 during Varied N.
Statistics for linear regression analysis of gel particle
numerical abundance and DOC concentration against organic matter components
during mesocosm experiments with different initial PO43- (Varied P; µmol L-1) and NO3- (Varied N; µmol L-1) concentrations.
Units – TEP: × 107 L-1; CSP: × 106 L-1; POC: µmol L-1; PN: µmol L-1; Chl a:
µg L-1;
DOC µmol L-1; bacteria: × 106 mL-1; a is the slope and b is the
intercept; n.s. stands for “not significant”. See Fig. 7 for further information.
Varied P
Varied N
a
b
n
r2
a
b
n
r2
TEP vs.
Chl a
2.0 ± 0.22
1.27 ± 0.39
60
0.65
1.2 ± 0.08
1.52 ± 0.26
96
0.69
POC
0.18 ± 0.012
-0.63 ± 0.49
89
0.72
0.22 ± 0.13
-1.5 ± 0.60
128
0.70
PN
1.2 ± 0.07
-1.4 ± 0.4
89
0.80
1.5 ± 0.1
-1.2 ± 0.7
128
0.64
DOC
0.14 ± 0.02
-8.7 ± 2.2
90
0.34
0.22 ± 0.02
-18 ± 2.1
127
0.54
Bacteria
3.4 ± 0.4
0.78 ± 0.67
90
0.46
6.8 ± 0.9
-1.3 ± 1.1
128
0.32
CSP vs..
Chl a
1.3 ± 0.1
1.4 ± 0.3
60
0.76
2.5 ± 0.15
3.2 ± 0.5
96
0.74
POC
0.11 ± 0.01
0.3 ± 0.4
89
0.58
0.25 ± 0.01
-1.2 ± 0.6
112
0.77
PN
0.7 ± 0.06
0.2 ± 0.3
89
0.65
1.9 ± 0.09
-0.1 ± 0.6
112
0.80
DOC
0.10 ± 0.01
-6.6 ± 1.4
90
0.42
0.21 ± 0.03
-13 ± 3.0
111
0.35
Bacteria
2.3 ± 0.26
0.63 ± 0.43
90
0.49
8.8 ± 0.84
-2.3 ± 1.11
128
0.47
DOC vs.
Chl a
n.s
–
–
–
n.s.
–
–
–
POC
0.54 ± 0.06
88 ± 2
119
0.38
0.66 ± 0.05
88 ± 2
127
0.58
PN
3.5 ± 0.4
88 ± 2
119
0.38
4,2 ± 0.4
90 ± 3
127
0.47
Bacteria
11 ± 1.5
92 ± 2
119
0.31
21 ± 3.1
88 ± 4
127
0.27
A direct coupling was also observed between gel particles and bacterial
abundance (Table 3). Like for POC and PN, the relative increase in gel
abundance was much steeper during Varied N than during Varied P, again showing that gel
particles in general were more abundant during the second experiment.
Although less pronounced than for particulate organic matter, TEP and CSP
numerical abundances were also related to DOC concentration during Varied P and
Varied N (Table 3), while no significant relationship was observed between gel
particle abundance and DON concentration. In contrast to gel particles,
however, DOC was not significantly related to Chl a concentration in both
experiments but rather to POC and PN concentrations (Fig. 7g–i, Table 3).
Differences in the relationship of DOC to POC or PN were relatively small,
suggesting an only slightly higher increase in DOC with particulates during
Varied N. DOC concentration correlated significantly with bacterial abundance (Table 3). The increase in DOC concentration relative to bacterial numbers was
almost twice as high during Varied N, suggesting that bacteria did not catch up with
DOC production during the second experiment.
Another comparison of both experiments can be made by relating gel particle
abundance to initial [DIN] : [DIP] ratios that covered a similar range during
both experiments (Fig. 8). This showed that, for similar initial nutrient
ratios, maximum abundances of both TEP and CSP were generally higher during
the second experiment, Varied N. Moreover, only during the second experiment did changes
in [DIN] : [DIP] ratios have an effect on maximum gel particle abundance.
The maximum numerical abundance of TEP (a) and CSP (b) in the mesocosms increased
with the initial (day 1) [DIN] : [DIP] ratio during
Varied N (open symbols), but not during Varied P (solid symbols).
However, direct comparison of the “centerpoint” treatment 12.0N / 0.75P that
was realized with four replicates during Varied P and three replicates during Varied N showed clear
differences in organic matter development during the two experiments for
mesocosms that received the same nutrient addition (Fig. 9). For this
treatment, Chl a concentration, DOC and TEP accumulated about 2 times more
in the course of Varied N, while the increase in CSP abundance over time was even
3-fold higher.
Discussion
Nutrient availability and phytoplankton bloom development
After fertilization with inorganic nutrients, phytoplankton blooms developed
in all mesocosms during the two consecutive experiments conducted with
natural surface water from the ETNA.
Responses to varied nutrient supply became more obvious after one (or both)
of the macronutrients was exhausted, resulting in a large variation in organic
matter concentration among mesocosms and treatments during the bloom peak
and post-bloom phases. Accumulation of organic matter during bloom
development revealed a generally stronger fertilization effect after
addition of different amounts of NO3- in the second experiment
compared to the first one with varied initial PO43- supply. This
indicates that biomass production in ETNA surface waters near Cape Verde may
be limited by nitrogen rather than by phosphorus availability. However,
clear differences between both experiments were also observed for mesocosms
receiving the same nutrient supply. This suggests that small differences in
the initial conditions of experiments with natural communities, such as
during this mesocosm study, can significantly impact the outcome of
biogeochemical responses.
Comparison of Chl a concentration (a), accumulation of
DOC (b, open symbols) and DON (b, solid symbols), and abundance of TEP (c,
open symbols) and CSP (c, solid symbols) observed in the course of the two
mesocosm experiments for the treatment 12.0N / 0.75P. Direct relationships
([yVariedN] =a [× VariedP]+b) were observed for
Chl a, with a= 2.3 ± 0.2, r2= 0.94, n= 8; DOC with
a= 2.1 ± 0.4, r2= 0.84, n= 8; TEP with a= 1.7 ± 0.4, r2= 0.78, n= 6; and CSP with
a= 3.3 ± 0.7, r2= 0.86, n= 6. Symbols represent mean values of
three mesocosms (Varied P) or four mesocosms (Varied N) with ±1 standard deviation (error
bars).
Moderate variations in responses of planktonic food webs and associated
biogeochemical cycling to the same nutrient treatment have been observed
previously for mesocosm experiments conducted at different marine ecosystem
sites, but a coherent picture of nitrogen stimulation was clearly
demonstrated (Olsen et al., 2006; Vadstein et al., 2012).
During this study, phytoplankton abundance was lower during the early days
of Varied N, while bacterial abundance was higher, despite sampling of initial
waters at the same location and within a time difference of only a few days.
Moreover, differences between Varied N and P were identified for the initial
community composition of diazotrophs (Meyer et al., 2015). We cannot fully
exclude that these differences in the initial conditions generally affected
the sensitivity to nutrient addition, regardless of the varied nutrient
concentration, and were also responsible for the higher response in mesocosms
where the same nutrient treatment was applied. However, the clear increase
in organic matter accumulation with increasing initial NO3-
concentration is in accordance with previous findings (Franz et al., 2012a)
and strongly suggests that ecosystems in the ETNA are controlled by
NO3- rather than by PO43- availability.
Ratios of estimated carbon content of transparent
exopolymer particles (TEP-C) to concentrations of particulate organic carbon
(POC), total organic carbon (TOC), particulate nitrogen (PN) and Chl a.
Average values (mean for TEP-C, median for ratios) are given for replicate
treatments on day 1 and 8 during Varied P and Varied N, respectively.
Sampling
n
Treat_ID
TEP-C
TEP-C : POC
TEP-C : TOC
TEP-C : PN
TEP-C : Chl a
[µmol L-1]
[mol %]
[mol %]
[mol : mol]
[µM : µg L-1]
Varied P
Day 1
3
12.0N / 0.25P
0.52
3.01
0.48
16.3
0.25
1
4
12.0N / 0.75P
0.40
3.12
0.36
3.4
0.26
1
1
6.35N / 1.10P
0.98
5.98
0.83
13.0
0.45
1
1
17.65N / 1.10P
0.52
4.14
0.45
7.5
0.28
1
3
12.0N / 1.25P
0.49
3.99
0.46
4.7
0.29
1
3
12.0N / 1.75P
0.55
5.12
0.50
4.4
0.38
Day 8
3
12.0N / 0.25P
13
41
8.4
2.21
25
8
4
12.0N / 0.75P
16
15
10.4
2.62
28
8
1
6.35N / 1.10P
21
28
14.8
2.85
63
8
1
17.65N / 1.10P
22
31
13.3
2.77
61
8
3
12.0N / 1.25P
18
26
9.9
2.79
32
8
3
12.0N / 1.75P
35
19
17.6
3.63
83
Varied N
Day 1
3
2.0N / 0.75P
0.36
3.07
0.29
0.21
2.04
1
3
4.0N / 0.75P
0.62
4.90
0.54
0.36
3.84
1
1
6.0N / 1.03P
0.50
4.99
0.48
0.41
4.18
1
1
6.35N / 0.40P
0.51
4.56
0.47
0.35
3.63
1
3
12.0N / 0.75P
0.88
7.34
0.90
0.66
5.16
1
1
17.65N / 0.40P
1.77
14
1.67
1.12
9.33
1
1
17.65N / 1.10P
0.97
9.32
0.91
0.66
7.43
1
3
20.0N / 0.75P
0.75
4.86
0.68
0.40
4.37
Day 8
3
2.0N / 0.75P
25
52
12.0
3.42
47
8
3
4.0N / 0.75P
46
82
22.4
6.61
48
8
1
6.0N / 1.03P
35
87
18.9
6.88
36
8
1
6.35N / 0.40P
37
53
17.3
4.03
45
8
3
12.0N / 0.75P
40
43
15.6
3.51
43
8
1
17.65N / 0.40P
68
93
29.1
6.36
81
8
1
17.65N / 1.10P
23
26
9.7
1.79
14
8
3
20.0N / 0.75P
42
47
16.1
3.16
19
It should be kept in mind that mesocosm experiments such as conducted during
this study can only capture a transient response to perturbation, such as
nutrient addition, and mainly give insights into short-term effects on
processes. Extrapolating from mesocosm experiments to long-term responses
of natural systems is not straightforward. Hence, although the response to
NO3- addition during the second experiment was pronounced, it
represents only one possible outcome. The observed differences for the
12.0N / 0.75P treatment indicate that the response of an ecosystem to nutrient
supply may vary even in a comparatively stable environment like the ETNA.
Clearly, a better knowledge of the impact of ecological variability, e.g.
plankton community structure, diversity and acclimation potential, on
biogeochemical processes is needed to fully explain differences in the
response size to perturbation.
Nutrient effects on gel particle dynamics
Previous studies on TEP and CSP in marine systems have suggested that the
rate of gel particle formation depends on the amount and chemical quality of
dissolved precursors (Engel et al., 2004a; Mari and Robert, 2008; Chow et
al., 2015). For extracellular organic matter released by bacterio- and
phytoplankton, the chemical composition and molecular weight of compounds
varies among species, and is also dependent on environmental conditions and
physiological status (Aluwihare and Repeta, 1999; Grossart et al., 2007;
Borchard and Engel, 2015). Because extracellular release is a major source
for gel particle precursors, factors influencing this release likely also
affect marine gel particle formation.
During this study, a clear accumulation of DOC was observed along with
biomass build-up in all mesocosms, indicating that the rate of DOC release
exceeded DOC loss processes such as coagulation into gel particles or
microbial uptake and respiration. Higher ΔDOC values were observed
shortly after the Chl a peak, coinciding with nutrient concentrations being
strongly reduced. Both enhanced extracellular release of DOC and “malfunctioning”
of the microbial loop, i.e. reduced microbial uptake and respiration of DOC
by bacteria, have been suggested to explain DOC accumulation in the ocean,
particularly at times when inorganic nutrients become depleted (Myklestad,
1974; Biddanda and Benner, 1997; Thingstad et al., 1997; Engel et al.,
2004b). During this study, DOC accumulation was significantly related to the
initial NO3- concentration during Varied N, suggesting a dependence on the
trophic status, although no direct relationship to Chl a concentration was
observed. Higher accumulation of DOC with increasing nitrogen load has been
observed during previous mesocosm experiments and explained by a combination
of production and recycling of DOC being higher at higher microbial
biomass (Vadstein et al., 2012). In addition, phosphorus limitation may have
reduced bacterial utilization of DOC in mesocosms with high initial
[DIN] : [DIP] ratios and below detection levels of PO43- after the
bloom, when highest accumulation rates of DOC occurred.
In contrast to DOC, no accumulation of DON was observed in the course of the
experiments in almost all mesocosms, except for the last day of Varied P and for
those treatments receiving highest NO3- additions during Varied N. This
indicates that loss processes such as microbial utilization of organic
nitrogen forms or partitioning of DON into CSP exceeded DON release during
this study.
In general, little is known about gel particle production at tropical open-ocean sites. To the best of our knowledge, this is the first report on TEP
and CSP abundance in the ETNA. Data on TEP-C concentration observed during
our study (Table 4), however, agree well with observations from Wurl et al. (2011), who determined < 2 to 40 µmol C L-1 for TEP in
surface waters of the tropical North Pacific (offshore of Hawaii).
Like DOC, gel particle abundance during this study was strongly related to
the general build-up and decay of autotrophic biomass. Thereby, a
significant impact of initial NO3- supply on gel particle
abundance, especially on CSP, was observed during Varied N, as well as a general
increase in the maximum abundance of gel particles with the initial
[DIN] : [DIP] ratio.
An increase in TEP formation, when phytoplankton was grown at higher
NO3- concentration, is in accordance with earlier observations
made during culture and mesocosm experiments (Corzo et al., 2000; Pedrotti
et al., 2010) and has been explained by higher biomass production at higher
nutrient loads; a larger biomass leads to a higher amount of released
polysaccharides when the autotrophic biomass runs into nutrient limitation.
A general relationship between TEP and autotrophic biomass concentration,
e.g. determined as Chl a, has been observed before (Passow, 2002; Beauvais et
al., 2003). Furthermore, species- or physiology-specific variations in TEP
formation by phytoplankton were observed (Berman and Viner-Mozzini, 2001;
Claquin et al., 2008; Passow, 2002). During this study, gel particles and
Chl a dynamics were decoupled after the Chl a peak. Hence, despite the general
observation that higher autotrophic biomass leads to more gel particles,
temporal developments of Chl a and gel particle concentration may contrast
between bloom build-up and decay. This is in accordance with earlier studies
showing that POC, TEP and CSP concentration continued to increase after the
Chl a peak (Alldrege et al., 1995; Engel, 2002; Logan et al., 1994; Mari and
Kiørboe, 1996) and can be explained by the “carbon-overflow” theory
(Schartau et al., 2007; Kreus et al., 2015).
During the post-bloom phase, a large proportion of organic carbon was thus
channeled to the POC pool in the form of TEP. Carbon contained in TEP
accounted for 0.5 µmol C (initial days) to ∼ 68 µmol C (final days), indicating a much higher DOC production than derived from
ΔDOC alone. The ratio of [TEP-C] : [POC] strongly increased during
Varied N, yielding values of up to 93 %. Even though these values are within the
range of earlier findings (Engel and Passow, 2001; Pedrotti, 2010), an
underestimation of POC, and hence overestimation of the [TEP-C] : [POC] ratio,
seems likely, because a large proportion of TEP was in the size range 1–2 µm ESD and may not be retained on GF/F filters
(nominal pore size 0.7 µm), due to their flexible and non-spherical structure (Engel and
Passow, 2001; Pedrotti, 2010). In addition, TEP-C calculation by use of an
empirical relationship to TEP size previously established from phytoplankton
cultures could overestimate carbon content of naturally occurring TEP at
this site.
A recent study by Rahav et al. (2013) suggested that bacterial diazotrophs
in aphotic, DIN-rich layers of the Red Sea and eastern Mediterranean Sea
benefit from TEP as an organic carbon source, resulting in an increase in
aphotic nitrogen fixation with TEP concentration. For the ETNA, unicellular
heterotrophic diazotrophs are readily abundant, also below the euphotic zone
in high-DIN waters (Langlois et al., 2005), and contribute substantially to
total nitrogen fixation of the system (Agawin et al., 2014). High TEP
production by surface phytoplankton communities in the ETNA as observed
during this study, and settling of TEP to aphotic layers, may therefore
provide an important labile carbon source for sustaining heterotrophic
nitrogen fixation.
While the importance of TEP formation for converting DOC to POC, and related
consequences for carbon cycling and export, has been highlighted over the
past decades (Alldrege et al., 1995; Passow, 2002; Engel et al., 2004a;
Arrigo, 2007), little is known about the role of CSP, in organic carbon and
more importantly in organic nitrogen cycling. It is likely that CSP play a
significant role for nitrogen cycling, contributing to DON to PN conversion
and to the PN pool as well as providing a nitrogenous resource for auto- and
heterotrophic growth. During this study, clearly higher accumulation of CSP
relative to Chl a was observed during Varied N, i.e. when a surplus of inorganic
nitrogen was available. As a consequence, CSP contributed more to POC and
PON increase during Varied N than during Varied P. This suggests that higher inorganic
nitrogen supply favors production of extracellular PON, which may be subject
to bacterial utilization at a later time. Because CSP are proteinaceous
particles, their export to depth, e.g. by physical transport or as part of
sinking aggregates, may provide important amino acids for microorganisms in
aphotic zones, including denitrifying and anammox bacteria. Since labile
amino acids have been suggested to be one important factor limiting organic
matter degradation in oxygen minimum zones (Pantoja et al., 2004, 2009), a
supply with CSP from the photic zone may also affect total carbon
remineralization and thus oxygen consumption at deeper depths. Our
results furthermore suggest that CSP as proteinaceous particles may include
an important fraction of organic nitrogen in the size fraction typically
attributed to bacteria.
Formation, aggregation and degradation of gel particles –
insights from size–frequency distributions
Changes in the size–frequency distribution of TEP during this study revealed
an increase in the proportion of larger particles in the course of
phytoplankton blooms, indicating TEP aggregation rather than degradation
during both experiments. For CSP, decreasing slopes together with a strong
increase in total abundance revealed an increasing number of smaller
particles during Varied N, indicating new formation of CSP during the phytoplankton
bloom and post-bloom periods. Occurrence of CSP at the time or depth of the
Chl a maxima has also been observed during previous studies (Cisternas-Novoa
et al., 2014, 2015). A clear indication of aggregation processes, i.e.
decreasing slopes as for TEP, was not observed for CSP. This is in
accordance with findings of Prieto et al. (2002), who suggested that CSP are
less involved in aggregate formation during diatom blooms than TEP. CSP
number and total area decreased at the end of Varied N (day 8), suggesting that loss
processes exceeded new CSP formation.
TEP and CSP both represent hotspots for microbial activity (Azam et al., 1983; Grossart et al., 1998; Bar-Zeev et al., 2009).
However, CSP are, as per definition, proteinaceous particles and thus expected to
include high amounts of labile N compounds. The observed decrease in CSP
abundance at day 8 of Varied N can therefore be explained by bacterial degradation
in order to liberate N, as suggested earlier (Long and Azam, 1996). Bacteria
cell numbers sharply increased from day 5 onwards during both experiments,
along with the strongest increase in gel particle abundance.