Effects of different plankton communities and spring bloom phases on seston C:N:P:Si:chl a ratios in the Baltic Sea

Abstract

Plankton communities and their temporal development have shifted towards earlier onset of the spring bloom and lower diatom-dinoflagellate proportions in parts of the Baltic Sea. We studied the effects of community composition and spring bloom phases on seston nutrient stoichiometry, revealing possible consequences of these shifts. Community composition, seston C:N:P:Si:chl a ratios, and physiological and environmental variables were determined for 4 research cruises, covering all major sub-basins and bloom phases. A redundancy analysis revealed that temperature and inorganic nutrients were the main drivers of community changes, and high diatom biomass was linked to low temperatures (growth phase). The effects of changing dominance patterns on seston stoichiometry were studied by applying a community ordination (non-metric multidimensional scaling and generalized additive models). C:N:P ratios increased from the growth phase (103:14:1) to the peak phase (144:18:1) and decreased after inorganic nitrogen was depleted (127:17:1). Taxonomic differences explained ~50% of changes in C:Si, N:Si, and chl a:C ratios and <30% for C:P and N:P, whereas C:N was virtually unaffected by the community composition. The fixed chl a:C range (~0.005-0.04) was largely determined by diatoms, independent of the dominant species. Thus, C:Si and N:Si could be used to estimate the share of diatoms to the seston and chl a:C to describe bloom phases and C budgets during spring. Interestingly, mixed communities featured higher C:N:P ratios than diatom-dominated ones. However, as community composition explained <30% of changes in C:N:P, we conclude that these ratios rather represent the total plankton physiology in natural plankton assemblages.

Food sources drive temporal variation in elemental stoichiometry of benthic consumers

“…C : P and N : P ratios were thus above the Redfield ratio of 106 : 16 : 1 (Redfield 1934), but close to the global median ratio where C : N, C : P, and N : P are 6.6, 163, and 22 (Martiny et al 2014). Moreover, during the peak of the spring bloom the average elemental content ratio of the phytoplankton in the Baltic Sea is 144 : 18 : 1 (Lipsewers et al 2020), whereas the corresponding values we measured for the benthic consumers in beginning of May were 138 : 20 : 1 (sheltered) and 148 : 21 : 1 (exposed). Benthic consumers depend heavily on pelagic production; and therefore, changes in the sinking organic matter may cause imbalances in resource availability.…”

“…Therefore, also the feeding dynamics of benthic fauna is characterized by seasonal patterns, high activity in summer vs. starvation in the winter (Coma et al 2000). Decreasing temperature is the main driver of these patterns, as it directly lowers the metabolic rate of the benthic animals (Brown et al 2004) and indirectly alters the productivity and composition of phytoplankton communities (Clarke 1993; Lipsewers et al 2020). During a long starvation period, the benthic animals burn body lipids for energy and lose carbon, which leads to decrease in C : N and C : P ratios, and increase in N content (Haubert et al 2004).…”

“…Hence, long‐term shifts in nutrient availability will most likely affect the tissue composition of these consumers. For instance, eutrophication and global warming have caused spring blooms to start earlier and changed the phytoplankton community composition, which in turn may alter the stoichiometry of the sinking organic matter and thus the food sources for the benthic consumers (Spilling et al 2014, 2018; Lipsewers et al 2020; Fig. 5).…”

Nutrient dynamics and coupling with biological processes and physical conditions in the Bohai Sea

“…Studies have shown that the optimum N/P ratio for growth uptake of cryptophytes is higher than 16 (Schöllhorn and Graneĺi, 1997), which may explain the higher DDIN/DDIP ratio in spring. Lipsewers et al (2020) investigated the effect of changes in dominance patterns on the stoichiometric ratio of phytoplankton in the Baltic Sea by applying community ordination (non-metric multidimensional scaling and generalized additive models). The results showed that the N/P ratios of phytoplankton ranged from 9 to 38 and increased with phytoplankton diversity.…”