Spring bloom community change modifies carbon pathways and C:N:P: Chl a stoichiometry of coastal material fluxes

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Abstract

Diatoms and dinoflagellates are major bloom-forming phytoplankton groups competing for resources in the oceans and coastal seas. Recent evidence suggests that their competition is significantly affected by climatic factors under ongoing change, modifying especially the conditions for cold-water, spring bloom communities in temperate and Arctic regions. We investigated the effects of phytoplankton community composition on spring bloom carbon flows and nutrient stoichiometry in multiyear mesocosm experiments. Comparison of differing communities showed that community structure significantly affected C accumulation parameters, with highest particulate organic carbon (POC) buildup and dissolved organic carbon (DOC) release in diatom-dominated communities. In terms of inorganic nutrient drawdown and bloom accumulation phase, the dominating groups behaved as functional surrogates. Dominance patterns, however, significantly affected C : N : P : Chl a ratios over the whole bloom event: when diatoms were dominant, these ratios increased compared to dinoflagellate dominance or mixed communities. Diatom-dominated communities sequestered carbon up to 3.6-fold higher than the expectation based on the Redfield ratio, and 2-fold higher compared to dinoflagellate dominance. To our knowledge, this is the first experimental report of consequences of climatically driven shifts in phytoplankton dominance patterns for carbon sequestration and related biogeochemical cycles in coastal seas. Our results also highlight the need for remote sensing technologies with taxonomical resolution, as the C : Chl a ratio was strongly dependent on community composition and bloom stage. Climate-driven changes in phytoplankton dominance patterns will have far-reaching consequences for major biogeochemical cycles and need to be considered in climate change scenarios for marine systems.

Figures

  • Table 1. Summary of the experimental setup in different years. The treatments were nutrient addition (NPSi), which were additions of nitrogen (N), phosphorus (P), and silicate (Si) in N–P, Si and N–P–Si additions. The light treatment (light) was a low- and high-light treatment, 20 and 90 µmolphotonsm−2 s−1, respectively. In 2005 there was only the nutrient addition treatment and in 2007 there cultured diatoms were added in a gradient (diatom gradient). The diatoms added were Thalassiosira levanderi (∼ 10 µm diameter and was added to a final concentration of 20 000, 75 000 and 13 350 cells L−1) and T. baltica (20–30 µm diameter and was added to a final concentration of 2000, 3560 and 13 350 cells L−1), two very typical spring bloom species. The start concentration of NO3 (Start NO3) gives the concentration in µgL−1 of NO3 in the control and in the treatments with N addition. The peak Chl a values are the minimum and maximum concentration recorded in the control (no nutrient addition) and in treatments with nutrients added, respectively.
  • Figure 1. An example of the data extracted from the mesocosms (data from high-light treatment with N and P addition, 2004): dissolved, inorganic nutrients and particulate, organic nutrients (a) and carbon parameters (b). The parameters are nitrate (NO3), particulate organic nitrogen (PON), phosphate (PO4), particulate organic phosphorus (POP), dissolved silicate (DSi), biogenic silicate (BSi), particulate organic carbon (POC), dissolved organic carbon (DOC), and total gross production (TGP). All parameters were measured directly except TGP, which was extrapolated from short-term 14C incubations. The growth was divided into exponential and stationary growth phases based on the primary production peak (L−1), indicated with the horizontal bars on top (b). Note the different scales on the y axes.
  • Figure 2. Species evenness plotted against the dinoflagellate proportion of the whole community. For later analysis, the phytoplankton community was divided into three categories: diatom dominance (> 80 %), mixed community (20–70 % dinoflagellates), and dinoflagellate dominance (> 70 %). The rationale behind setting the group boundaries was based upon the apparent difference in species evenness.
  • Figure 3. The growth rate during exponential growth of particulate organic carbon (µPOC) and Chlorophyll a (µChl a) at different dinoflagellate proportion of the total phytoplankton community (a), and the relationship between carbon assimilation efficiency andµPOC (b). No significant trend was found for µPOC, but a negative correlation was found between µChl a and dinoflagellate proportion. The solid line represents the linear regression (slope=−0.16; R2 = 0.53; p < 0.0001), and the dashed lines represent the 95 % confidence intervals. The carbon assimilation efficiency is the ratio between the measured growth rate in POC and the total gross production (Fig. 1). A positive correlation was found (slope= 1.49; R2 = 0.12; p = 0.04).
  • Figure 4. Carbon budget parameters: particulate organic carbon (POC), dissolved organic carbon (DOC), and total gross production, during exponential and stationary growth. The phytoplankton community was divided into three categories: diatom dominance (> 80 %), mixed community (20–70 % dinoflagellates), and dinoflagellate dominance (> 70 %). The rationale behind setting the group boundaries was based upon the apparent difference in species evenness (Fig. 2) between these groups. The stars (*) indicate statistical significance (α = 0.05) against one (*) or two groups (**); details can be found in Tables 3 and 4.
  • Table 3. Tukey’s post hoc tests of carbon parameters during exponential growth phase. Only the statistically significant parameters from Table 2 were tested: carbon assimilation efficiency (CAE) and respiration (RES). The phytoplankton community was categorized according to diatom dominance (diatoms), mixed community (mixed) and dinoflagellate dominance (dinoflagellates). The stars (∗) indicate statistically significant differences (α = 0.05).
  • Table 2. Statistical comparison using one-way ANOVA on ranks. Tukey’s post hoc test of statistically significant differences (∗) can be found in Table 3 (exponential growth phase) and 4 (stationary growth phase).
  • Figure 5. Carbon budget parameters: carbon assimilation efficiency and respiration, during exponential and stationary growth. The phytoplankton community was divided into three categories: diatom dominance (> 80 %), mixed community (20–70 % dinoflagellates), and dinoflagellate dominance (> 70 %). The rationale behind setting the group boundaries was based upon the apparent difference in species evenness (Fig. 1) between these groups. The stars indicate statistically significant differences (α = 0.05) compared to one (*) or two (**) other groups; details can be found in Table 4.

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Spilling, K., Kremp, A., Klais, R., Olli, K., & Tamminen, T. (2014). Spring bloom community change modifies carbon pathways and C:N:P: Chl a stoichiometry of coastal material fluxes. Biogeosciences, 11(24), 7275–7289. https://doi.org/10.5194/bg-11-7275-2014

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