Description: Highlights: • We used indoor mesocosms to test the impact of warming on plankton communities. • Different stages of phytoplankton bloom were analysed. • Increased temperature and zooplankton grazing had similar effects on phytoplankton. • Warming and increased zooplankton density decreased phytoplankton richness. • Warming and increased zooplankton density increased phytoplankton evenness. Recent climate warming is expected to affect phytoplankton biomass and diversity in marine ecosystems. Temperature can act directly on phytoplankton (e.g. rendering physiological processes) or indirectly due to changes in zooplankton grazing activity. We tested experimentally the impact of increased temperature on natural phytoplankton and zooplankton communities using indoor mesocosms and combined the results from different experimental years applying a meta-analytic approach. We divided our analysis into three bloom phases to define the strength of temperature and zooplankton impacts on phytoplankton in different stages of bloom development. Within the constraints of an experiment, our results suggest that increased temperature and zooplankton grazing have similar effects on phytoplankton diversity, which are most apparent in the post-bloom phase, when zooplankton abundances reach the highest values. Moreover, we observed changes in zooplankton composition in response to warming and initial conditions, which can additionally affect phytoplankton diversity, because changing feeding preferences of zooplankton can affect phytoplankton community structure. We conclude that phytoplankton diversity is indirectly affected by temperature in the post-bloom phase through changing zooplankton composition and grazing activities. Before and during the bloom, however, these effects seem to be overruled by temperature enhanced bottom-up processes such as phytoplankton nutrient uptake.

Description: The factors regulating species diversity have received increasing attention in the face of the global biodiversity loss, but are not well understood for unicellular organisms. We conducted in situ experiments in Kiel Fjord in order to analyze the response of microalgal diversity to colonization time and to artificial eutrophication. Diversity decreased throughout colonization time (maximum: 12 weeks), whereas species richness initially increased to about 25 species before it leveled off. The proposed unimodal time course of diversity during succession could not be detected for diversity or species richness. The rapid decrease of evenness indicated a greater importance of algal growth on the substrata compared to the arrival of new species. Artificial eutrophication led to an decrease of diversity, which could be correlated to the supply concentrations of the limiting nutrient: P in spring, N in summer and Si in the presence of high concentrations of N and P. The decrease was due to an increased dominance of few species (i.e. reduced evenness), whereas species richness was not or positively correlated to nutrient supply. Species richness was negatively correlated to evenness and diversity measures. Thus, species diversity indices are useful response variable to measure environmental effects on local periphyton communities

Description: In order to examine the effects of warming and diversity changes on primary productivity, we conducted a meta-analysis on six independent indoor mesocosm experiments with a natural plankton community from the Baltic Sea. Temperature effects on primary productivity changed with light intensity and zooplankton density and analysed pathways between temperature, diversity and productivity, elucidating direct and indirect effects of warming on primary productivity during the spring phytoplankton bloom. Our findings indicate that warming directly increased carbon specific primary productivity, which was more pronounced under low grazing pressure. On the other hand, primary productivity per unit water volume did not respond to increased temperature, because of a negative temperature effect on phytoplankton biomass. Moreover, primary productivity response to temperature changes depended on light limitation. Using path analysis, we tested whether temperature effects were direct or mediated by warming effects on phytoplankton diversity. Although phytoplankton species richness had a positive impact on both net primary productivity and carbon specific primary productivity – and evenness had a negative effect on net primary productivity – both richness and evenness were not affected by temperature. Thus, we suggest that diversity effects on primary productivity depended mainly on other factors than temperature like grazing, sinking or nutrient limitation, which themselves are temperature dependent. Highlights ► Impact of warming on primary productivity and diversity–productivity relationship. ► Meta-analysis on indoor mesocosm experiments with a natural plankton community. ► Temperature has a direct impact on specific productivity, not on net productivity. ► Species richness increases and evenness decreases net primary productivity. ► Temperature does not directly affect diversity–productivity relationship.

Description: Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C: N: P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C: N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C: N ratio ranged uniformly around 7.5. N: P ratios 〈13 indicated N limitation, while N: P ratios 〉22 indicated P limitation. Under P limitation, the C: P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C: N: P = 119 : 17 : 1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106 : 16 : 1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.

Description: Field experiments were conducted to investigate the effects of grazing and nutrient supply on sediment microflora in a freshwater habitat (Lake Erken, Sweden) and at the brackish Baltic Sea coast (Väddö, Sweden). The two sites were of similar productivity, but had contrasting herbivore composition. In a full-factorial experiment design, closed cages excluded macrozoobenthos (〉1 mm) from sediment patches, whereas open cages allowed grazer access. The cage design applied here proved to successfully prevent in- and epifauna to access the sediment in closed cages. In half of the treatments, nutrients were added to the water-column by a slow-release fertilizer. The experiments were seasonally replicated four times at Väddö and two times in Lake Erken. After 4–5 weeks, sediment cores were sampled and analyzed for chlorophyll, carbon, nitrogen and phosphorus. The benthic microalgae showed strong seasonal variation in biomass and internal nutrient content. At Väddö, neither grazing nor nutrients affected the algal biomass significantly, but significant grazer effects were detected on C:N:P ratios. In Lake Erken, grazer presence reduced algal biomass by ca. 50%, whereas nutrients were without effect on biomass or on nutrient content. Compared to results from hard substrata at the same sites, sediment microflora was less affected by nutrients and grazing. This may be due to the harsh physico-chemical environment on sediments, to low grazer density at the coastal site and to low availability of water column nutrients to sediment microalgae. In our experiments, sand-dwelling microphytobenthic communities represented a highly dynamic assemblage, which, however, is less structured by biotic interactions than epilithic periphyton

Description: We established a new indoor mesocosm facility, 12 fully controlled “Planktotrons”, designed to conduct marine and freshwater experiments for biodiversity and food web approaches using natural or artificial, benthic or planktonic communities. The Planktotrons are a unique and custom-tailored facility allowing long-term experiments. Wall growth can be inhibited by a rotating gate paddle with silicone lips. Additionally, temperature and light intensity are individually controllable for each Planktotron and the large volume (600 L) enables high-frequency or volume-intense measurements. In a pilot freshwater experiment various trophic levels of a pelagic food web were maintained for up to 90 d. First, an artificially assembled phytoplankton community of 11 species was inoculated in all Planktotrons. After 22 d, two ciliates were added to all, and three Daphnia species were added to six Planktotrons. After 72 d, dissolved organic matter (DOM, an alkaline soil extract) was added as an external disturbance to six of the 12 Planktotrons, involving three Planktotrons stocked with Daphnia and three without, respectively. We demonstrate the suitability of the Planktotrons for food web and biodiversity research. Variation among replicated Planktotrons (n = 3 minimum) did not differ from other laboratory systems and field experiments. We investigated population dynamics and interactions among the different trophic levels, and found them affected by the sequence of ciliate and Daphnia addition and the disturbance caused by addition of DOM.

Description: A central concept for understanding the mechanisms linking diversity and primary production or more general, ecosystem functioning, is resource use efficiency (RUE). It quantifies the amount of biomass production over time relative to unit resource supplied, that is, represents a quota of matter use efficiency. Given anthropogenic alterations of biogeochemical cycles, the consequent changes in supply rate and especially supply ratio of nutrients will change. Using four species of freshwater phytoplankton, and their mixture, we asked how the RUE for nitrogen and phosphorus depends on the stoichiometry of resource supply and how this differs between single species and their mixture. We conducted a factorial laboratory experiment spanning 25 different nutrient supply treatments with differing absolute and relative nitrogen (N) and phosphorus (P) concentrations. N and P supply increased biomass production and decreased C : nutrient ratios and RUE for the respective nutrient, but always significantly affected by the supply of the respective other nutrient. Biomass peaked at molar N : P supply ratios above the Redfield ratio (18-22). Species tended to respond similarly to the resource gradients. Consequently, mixtures outperformed the component species only during early growth responses, but not regarding maximum biomass and RUE. Bioassays performed at the end of the main experiment revealed predominance of N-limitation, but again strongly depending on the interaction between both nutrient gradients. Our study suggests that stoichiometric constraints of resource incorporation and RUE need to be accounted for when studying the response of phytoplankton to natural and anthropogenic variation in resource availability.