Benthic diatoms are dominant primary producers in intertidal mudflats and constitute a major source of organic carbon to consumers and decomposers residing within these ecosystems. They typically form biofilms whose species richness, community composition and productivity can vary in response to environmental drivers and their interactions with other organisms (e.g., grazers). Here, we investigated whether bacteria can affect diatom community composition and vice versa, and how this could influence the biodiversity-productivity relation. Using axenic experimental communities with three common benthic diatoms (Cylindrotheca closterium, Navicula phyllepta, and Seminavis robusta), we observed an increase in algal biomass production in diatom co-cultures in comparison to monocultures. The presence of bacteria decreased the productivity of diatom monocultures while bacteria did not seem to affect the overall productivity of diatoms grown in co-cultures. The effect of bacteria on diatom growth, however, appeared to be species-specific, resulting in compositional shifts when different diatom species were grown together. The effect of the diatoms on the bacteria also proved to be species-specific as each diatom species developed a bacterial community that differed in its composition. Together, our results suggest that interactions between bacteria and diatoms residing in mudflats are a key factor in the structuring of the benthic microbial community composition and the overall functioning of that community.

Alpha and beta diversity of harpacticoid copepods was studied in a Kenyan seagrass bed (Gazi Bay, Kenya) with a clear zonation of different seagrass species. The application of an appropriate sampling strategy made the interpretation of different spatial diversity levels possible. Alpha diversity was defined as the diversity of harpacticoid copepods associated with 1 seagrass species or 1 subhabitat (roots or leaves). Beta diversity was interpreted as changes in diversity between both subhabitats of 1 seagrass species and between different seagrass species along the tidal gradient. A total of 115 harpacticoid copepod species were recorded in the seagrass samples. Of these, 36 species (31.3%) were restricted to the root subhabitat and 12 (10.4%) were only recovered from leaf samples. Higher diversity was recorded for the deeper seagrass species (Syringodium isoetifolium, Halophila stipulacea). Copepod communities associated with Halophila ovalis and H. stipulacea (both pioneer seagrass species) were clearly different from one another in terms of diversity. A trend towards more specialized habitat preference (i.e. a lower ecological range size) was found with increasing diversity. The left-skewed species' range size distribution for the more diverse samples was clearly different from the typical right-skewed curves reported in most terrestrial studies. This may provide evidence for fundamental differences between marine species and terrestrial ones in their range size distribution. KEY WORDS: Harpacticoid copepods · Seagrass · Diversity · Ecological range sizeResale or republication not permitted without written consent of the publisher

While different microalgae tend to be associated with different bacteria, it remains unclear whether such specific associations are beneficial for the microalgae. We assessed the impact of bacterial isolates, derived from various marine benthic diatoms, on the growth of several strains belonging to the Cylindrotheca closterium diatom species complex. We first tested the effect of 35 different bacterial isolates on the growth of a single C. closterium strain, and then evaluated the impact of 8 of these isolates on the growth of 6 C. closterium strains and 1 Cylindrotheca fusiformis strain. Surprisingly, most interactions were neutral to antagonistic. The interactions were highly specific, with diatom growth in the presence of specific bacteria differing between Cylindrotheca strains and species, and closely related bacteria eliciting contrasting diatom growth responses. These differences could be related to the origin of the bacterial isolates, as only isolates from foreign diatom hosts significantly reduced diatom growth, implying coadaptation between different Cylindrotheca strains and their associated bacteria. Interestingly, the antagonistic effect of a Marinobacter strain was alleviated by the presence of a microbial inoculum that was native to the diatom host, suggesting that coadapted bacteria might also benefit their host indirectly by preventing the establishment of harmful bacteria.

Four common intertidal harpacticoid species (Paramphiascella fulvofasciata, Tigriopus brevicornis, Nitocra spinipes and Harpacticus obscurus) were offered pelagic diatoms (Phaeodactylum tricornutum) as food in laboratory experiments. To ensure generality we used copepod species originating from diverse habitats with different ecological characteristics. The diatoms were enriched in the stable carbon 13 C isotope to facilitate tracing in the harpacticoids. Uptake of diatoms was clearly species-specific and in general P. fulvofasciata was more efficient than the other species. We found significant uptake by 24 h of incubation for T. brevicornis. From 24 h onwards we found an increase for all 3 species. Species had similar δ 13 C values before and after starvation, indicating that labeled material was efficiently assimilated in their tissues. We tested whether diatom assimilation is density-dependent and this was true for 2 species (N. spinipes and P. fulvofasciata) but not for the semi-pelagic T. brevicornis. Finally, a positive effect of faecal pellets on food uptake for the less mobile species was shown, indicating that microbial gardening occurs within benthic harpacticoids as it does for several other crustacean species.

Knowledge of the food web structure and complexity are central to better understand ecosystem functioning. A food-web approach includes both species and energy flows among them, providing a natural framework for characterizing species’ ecological roles and the mechanisms through which biodiversity influences ecosystem dynamics. Here we present for the first time a high-resolution food web for a marine ecosystem at Potter Cove (northern Antarctic Peninsula). Eleven food web properties were analyzed in order to document network complexity, structure and topology. We found a low linkage density (3.4), connectance (0.04) and omnivory percentage (45), as well as a short path length (1.8) and a low clustering coefficient (0.08). Furthermore, relating the structure of the food web to its dynamics, an exponential degree distribution (in- and out-links) was found. This suggests that the Potter Cove food web may be vulnerable if the most connected species became locally extinct. For two of the three more connected functional groups, competition overlap graphs imply high trophic interaction between demersal fish and niche specialization according to feeding strategies in amphipods. On the other hand, the prey overlap graph shows also that multiple energy pathways of carbon flux exist across benthic and pelagic habitats in the Potter Cove ecosystem. Although alternative food sources might add robustness to the web, network properties (low linkage density, connectance and omnivory) suggest fragility and potential trophic cascade effects.