Abstract Understanding how global warming-induced changes in phytoplankton affect nutrient cycling is essential for assessing climate change impacts in freshwater environments. The present study used a multispecies model to explore the effect of temperature on nutrient uptake efficiency and its link with community structure and resource use-related traits. Simulations were run under four different annual temperature patterns (baseline to +3°C), along various nutrient loads representing a nitrogen-to-phosphorus (N:P) ratio gradient, and under constant and fluctuating supply. Warming led to more balanced annual nutrient uptake and increasing mean uptake efficiency. Under constant loads, this increase depended on the N:P ratio and on compositional shifts. In communities exhibiting high species turnover under warming, increase in nutrient uptake was constrained by higher minimum resource requirements (R*) during the nutrient-limited summer period. Higher fluctuations in nutrient loads enhanced warming-induced increases in uptake efficiency, independently of shifts in R*, also increasing annual net growth time. These results suggest that warming impacts on nutrient uptake can be traced back to multiple mechanisms: trait-mediated changes in competitive ability, and changes in the length of the growth period. Thus, the relative importance of community reassembly and environmental variability might have substantial influence on phytoplankton-mediated ecosystem responses to climate change.
Abstract Benthic copepods inhabit complex boundary layers where flow varies rapidly, yet their sensory-motor responses to localized hydrodynamic signals remain poorly understood. We quantified the behavior of the harpacticoid Tigriopus californicus under no-flow and localized suction flow using 3D particle tracking velocimetry of both copepod trajectories and the surrounding flow. Copepods frequently entered the water column and maintained a hop-and-sink gait, spending ~40% of time jumping with peak speeds up to 68 mm s−1. In the far field (>2 mm from the source), kinematic statistics were indistinguishable from no-flow controls and orientations were consistent with random swimming. In the near field, individuals performed directed escape jumps (mean 80.6 ± 17.9 mm s−1; max. 105.7 mm s−1). Escape onset occurred at relative speeds (≈ 6 mm s−1) and strain rates (≈ 5 s−1) characteristic of localized deformation, at distances from the source on the order of the organism size. These results reveal a context-dependent modulation of locomotion that preserves routine hop-and-sink gait under weak flow while enabling rapid evasion near localized disturbances. The findings are consistent with gradient-based mechanosensation, although direct contact with the suction inlet may have contributed to some escapes. Broader generalization will require replication across taxa, life stages and flow regimes.
Hydrological variation in river-floodplain systems is associated with changes in environmental heterogeneity and dispersal-related processes. Using data from a 15-year monitoring program in the Upper Paran & aacute; River floodplain, we quantified the environmental and spatial fractions of zooplankton community variation across taxonomic groups and contrasting hydrological periods. Patterns associated with metacommunity organization varied consistently among zooplankton groups along the hydrological cycle. As river level increased, fine-scale spatial fractions compatible with mass effects became more evident, especially during high-water periods, suggesting increased spatial signals associated with dispersal among habitats. In contrast, during low-water conditions, environmental fractions consistent with species sorting were generally higher, particularly for smaller-bodied taxa. However, cladocerans exhibited a non-linear response, with higher environmental factors contributing at intermediate water levels. Large-scale spatial fractions consistent with dispersal limitation were more evident for copepods during low-water periods, suggesting increased spatial constraints under reduced connectivity. Overall, our results indicate that hydrological variation is associated with shifts in environmental and spatial signals in zooplankton metacommunities, and these signals differ among taxonomic groups.
Phytoplankton and microzooplankton (MZP) are key components of marine food webs, driving carbon transfer through the microbial loop. Short-term laboratory microcosm bottle incubations using seawater from the Bay of Bengal (BoB) and Arabian Sea (AS) in 2022 examined the effects of reduced pH. Phytoplankton biomass (Chl-a) remained stable or increased by up to 12% under low pH, suggesting reduced grazing or improved prey quality. MZP diversity (Shannon index, H ') showed basin-specific responses. In the BoB (Exps. 1-2), H ' ranged 2.00-3.70 with high evenness (J ' = 0.93-1.00) and slight declines under medium acidification (Delta pH = 0.2). In contrast, the AS (Exps. 3-4) showed wider variability (H ' = 0.00-3.63) and greater sensitivity, with sharp diversity losses under stronger acidification (Delta pH = 0.4). AS assemblages showed pronounced richness declines (D ' = 2.85-0.00), whereas BoB communities remained stable. Regional and pH-related differences were linked to shifts in ciliates and dinoflagellates. Autotrophic bacteria sustained under low pH, while heterotrophic bacteria increased later, indicating altered microbial-loop functioning. Overall, pH, chlorophyll-a, and heterotrophic bacteria structured the community, accounting for > 65% of the variation. Thus, short-term acidification reshapes MZP assemblages and may modify trophic interactions across the northern Indian Ocean.
