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: To examine the grazing effects of copepod-dominated mesozooplankton on heterotrophic microbial communities, four mesocosm experiments using gradients of zooplankton abundance were carried out at a coastal marine site. The responses of different protist groups (nanoflagellates, ciliates) and bacterioplankton in terms of abundance and additionally, for bacteria, diversity, production, and exoenzymatic activity, were monitored during 1 week of incubation. Independent of the initial experimental abiotic conditions and the dominating copepod species, zooplankton caused order-of-magnitude changes in microbial functional groups in a clear community-wide four-link trophic cascade. The strongest predatory effects were observed for protist concentrations, thus generating inverse relationships between mesozooplankton and ciliates and between ciliates and nanoplankton. Copepod grazing effects propagated even further, not only reducing the abundance, production, and hydrolytic activity of bacterioplankton but also increasing bacterial diversity. The overall strength of this trophic cascade was dampened with respect to bacterial numbers, but more pronounced with respect to bacterial diversity and activity. High predation pressure by heterotrophic nanoflagellates, realized at the highest copepod abundance, was probably the underlying mechanism for these structural changes in the bacterial assemblages. Our results thus suggest a mechanism by which changes in higher trophic levels of marine plankton indirectly affect prokaryotic assemblages and microbially mediated ecosystem functions.

Description: The seasonal development of bacteria was studied in the hypertrophic coastal lagoon Ciénaga Grande de Santa Marta (Caribbean coast of Colombia). This large but only 1.5 m deep lagoon is subject to strong seasonal variations of salinity from almost fully marine (April/May) to brackish conditions in October/November. Chlorophyll ranged from 6 to 182 μg L−1, and gross primary production amounted to 1690 g C m−2 per year. Total bacterial number (TBN) ranged from 6.5 to 90.5 × 109 cells L−1 and bacterial biomass (BBM) from 77 to 1542 μg C L−1, which are among the highest ever reported for natural coastal waters. Neither TBN nor BBM varied significantly with salinity, phytoplankton or seston concentrations. Only the bacterial mean cell volume showed a significant relation to salinity, being highest (0.066 μm3) during the period of increasing and lowest (0.032 μm3) during decreasing salinity. Bacterial protein accounted for 24% (19–26%) and phytoplankton protein for 57% (53–71%) of total seston protein. The ratio (annual mean) of bacterial carbon to phytoplankton carbon was 0.44 (range 0.04–1.43). At low phytoplankton abundance [chlorophyll a (Chl a) 〈 25 μg L−1], bacterial carbon was almost equal to phytoplankton biomass (i.e. the mean ratio was 1.04). In contrast, at Chl a 〉 100 μg L−1, BBM was low compared to phytoplankton biomass (the mean ratio was 0.16). In general, BBM varied less than phytoplankton biomass. Most probably, the missing correlation between bacterial and phytoplankton variables was due to (i) organic material partly derived from allochthonous sources serving as food resource for bacteria and (ii) a strong resuspension of bacteria from the sediment caused by frequent wind-induced mixing of the very shallow lagoon.