Rising temperature and marine plankton community dynamics: Is warming bad?

Warming increases the compositional and functional variability of a temperate protist community

Phototrophic protists are a fundamental component of the world's oceans by serving as the primary source of energy, oxygen, and organic nutrients for the entire ecosystem. Due to the high thermal seasonality of their habitat, temperate protists could harbour many well-adapted species that tolerate ocean warming. However, these species may not sustain ecosystem functions equally well. To address these uncertainties, we conducted a 30-day mesocosm experiment to investigate how moderate (12 °C) and substantial (18 °C) warming compared to ambient conditions (6 °C) affect the composition (18S rRNA metabarcoding) and ecosystem functions (biomass, gross oxygen productivity, nutritional quality - C:N and C:P ratio) of a North Sea spring bloom community. Our results revealed warming-driven shifts in dominant protist groups, with haptophytes thriving at 12 °C and diatoms at 18 °C. Species responses primarily depended on the species' thermal traits, with indirect temperature effects on grazing being less relevant and phosphorus acting as a critical modulator. The species Phaeocystis globosa showed highest biomass on low phosphate concentrations and relatively increased in some replicates of both warming treatments. In line with this, the C:P ratio varied more with the presence of P. globosa than with temperature. Examining further ecosystem responses under warming, our study revealed lowered gross oxygen productivity but increased biomass accumulation whereas the C:N ratio remained unaltered. Although North Sea species exhibited resilience to elevated temperatures, a diminished functional similarity and heightened compositional variability indicate potential ecosystem repercussions for higher trophic levels. In conclusion, our research stresses the multifaceted nature of temperature effects on protist communities, emphasising the need for a holistic understanding that encompasses trait-based responses, indirect effects, and functional dynamics in the face of exacerbating temperature changes.

Major declines in NE Atlantic plankton contrast with more stable populations in the rapidly warming North Sea

Plankton form the base of marine food webs, making them important indicators of ecosystem status. Changes in the abundance of plankton functional groups, or lifeforms, can affect higher trophic levels and can indicate important shifts in ecosystem functioning. Here, we extend this knowledge by combining data from Continuous Plankton Recorder and fixed-point stations to provide the most comprehensive analysis of plankton time-series for the North-East Atlantic and North-West European shelf to date. We analysed 24 phytoplankton and zooplankton datasets from 15 research institutions to map 60-year abundance trends for 8 planktonic lifeforms. Most lifeforms decreased in abundance (e.g. dinoflagellates: -5 %, holoplankton: -7 % decade-1), except for meroplankton, which increased 12 % decade-1, reflecting widespread changes in large-scale and localised processes. K-means clustering of assessment units according to abundance trends revealed largely opposing trend direction between shelf and oceanic regions for most lifeforms, with North Sea areas characterised by increasing coastal abundance, while abundance decreased in North-East Atlantic areas. Individual taxa comprising each phytoplankton lifeform exhibited similar abundance trends, whereas taxa grouped within zooplankton lifeforms were more variable. These regional contrasts are counterintuitive, since the North Sea which has undergone major warming, changes in nutrients, and past fisheries perturbation has changed far less, from phytoplankton to fish larvae, as compared to the more slowly warming North-East Atlantic with lower nutrient supply and fishing pressure. This more remote oceanic region has shown a major and worrying decline in the traditional food web. Although the causal mechanisms remain unclear, declining abundance of key planktonic lifeforms in the North-East Atlantic, including diatoms and copepods, are a cause of major concern for the future of food webs and should provide a red flag to politicians and policymakers about the prioritisation of future management and adaptation measures required to ensure future sustainable use of the marine ecosystem.

Planktonic ecological networks support quantification of changes in ecosystem health and functioning

Plankton communities are the foundation of marine food webs and have a large effect on the dynamics of entire ecosystems. Changes in physicochemical factors strongly influence planktonic organisms and their turnover rates, making their communities useful for monitoring ecosystem health. We studied and compared the planktonic food webs of Palude della Rosa (Venice Lagoon, Italy) in 2005 and 2007. The food webs were developed using a novel approach based on the Monte Carlo random sampling of parameters within specific and realistic ranges to derive 1000 food webs for July of each year. The consumption flows involving Strombididae, Evadne spp. and Podon spp. were identified as the most important in splitting food webs of the July of the two years. Although functional nodes (FNs) differed both in presence and abundance in July of the two years, the whole system indicators showed very similar results. Sediment resuspension acted as a source of stress for the Venice Lagoon, being the most used resource by consumers while inhibiting primary producers by increasing water turbidity. Primary production in the water column was mainly generated by benthic FNs. Although the system was near an equilibrium point, it tended to increase its resilience at the expense of efficiency due to stress. This study highlights the role of plankton communities, which can serve to assess ecosystem health.

The investigation of the zooplankton community in the newly formed Ribb Reservoir, Ethiopia: the tropical highland reservoir

Understanding the composition, diversity, and abundance of the zooplankton community is crucial for better utilization of the Ribb Reservoir, as zooplankton are the second link in the food chain in aquatic systems (they are also excellent bioindicators of aquatic health, given their central food web position) and the reservoir also serves as a source of income for the fishers. Therefore, sampling including some water quality parameters was conducted twice in the four seasons: autumn, summer, spring, and winter, from September 2020 to August 2021, in the first week of September, December, February, March, May, June, and August. Most of the physicochemical parameter values recorded in this study indicated that they were within the range of standards for zooplankton community requirements. Of the 14 species identified, Mesocyclops aequatorialissimilis, Thermodiaptomus galebi, and Brachionus angularis had the first, second, and lowest records, respectively. Species abundance showed a decrease from autumn to winter and then to spring and summer. Species richness (14), abundance (6736), Margalef's diversity index (1.48), Menhinick's diversity index (0.17), Simpson index (0.098), dominance index (0.902), Shannon index (2.47), equality index (0.934), and reciprocal Simpson index (10.2) of the species were calculated in the reservoir. Some of the proposed management measures include reservoir buffering, impact assessment of over-abstraction of water for irrigation, time series of water quality data, and the reservoir water level should be above the conduit.

Phase tipping: how cyclic ecosystems respond to contemporary climate

We identify the phase of a cycle as a new critical factor for tipping points (critical transitions) in cyclic systems subject to time-varying external conditions. As an example, we consider how contemporary climate variability induces tipping from a predator–prey cycle to extinction in two paradigmatic predator–prey models with an Allee effect. Our analysis of these examples uncovers a counterintuitive behaviour, which we call phase tipping or P-tipping, where tipping to extinction occurs only from certain phases of the cycle. To explain this behaviour, we combine global dynamics with set theory and introduce the concept of partial basin instability for attracting limit cycles. This concept provides a general framework to analyse and identify easily testable criteria for the occurrence of phase tipping in externally forced systems, and can be extended to more complicated attractors.

Depicting the seasonal and spatial sensitivity of anthropogenic nutrient enrichment on phytoplankton in the Bay of Bengal, India

High nutrient loading discharge into the oceans is an evolving threat to the marine biota at large, including the Bay of Bengal (BoB) which receives one of the highest inflows of nutrient fluxes from rivers like the Ganga and the Brahmaputra. Further, the complex riverine and deltaic formations are difficult to be quantified, hence satellite-based observation of chlorophyll and phytoplankton species can add more insight in the nutrient aggregation process. We evaluated the spatial sensitive zones in the BoB in terms of anthropogenic nutrients. Our study shows that coccolithophore population and chlorophyll a concentration can be an active indicator of flood mediated nutrient inflow into the oceans. SST illustrated weaker correlation with phytoplankton species during the monsoon owing to prevalence of cyclonic conditions and high nutrient concentration, however, the relationship is relatively stronger during the pre-monsoon when more stable oligotrophic conditions exist. Higher concentration of diatoms, cyanobacteria and chlorophytes in BOB compared to the Pacific Ocean indicates the ecological sensitivity and the resilience of the Bay of Bengal in terms of nutrient cycling.