The role of seagrass vegetation and local environmental conditions in shaping benthic bacterial and macroinvertebrate communities in a tropical coastal lagoon.

We investigated the influence of seagrass canopies on the benthic biodiversity of bacteria and macroinvertebrates in a Red Sea tropical lagoon. Changes in abundance, number of taxa and assemblage structure were analyzed in response to seagrass densities (low, SLD; high, SHD; seagrasses with algae, SA), and compared with unvegetated sediments. Biological and environmental variables were examined in these four habitats (hereafter called treatments), both in the underlaying sediments and overlaying waters, at three randomly picked locations in March 2017. Differences between treatments were more apparent in the benthic habitat than in the overlaying waters. The presence of vegetation (more than its cover) and changes in sedimentary features (grain size and metals) at local scales influenced the observed biological patterns, particularly for macroinvertebrates. Of note, the highest percentage of exclusive macroinvertebrate taxa (18% of the gamma diversity) was observed in the SHD treatment peaking in the SA for bacteria. Benthic macroinvertebrates and bacteria shared a generally low number of taxa across treatments and locations; approximately, 25% of the gamma diversity was shared among all treatments and locations for macrofauna, dropping to 11% for bacteria. Given the low overlap in the species distribution across the lagoon, sustaining the connectivity among heterogeneous soft sediment habitats appears to be essential for maintaining regional biodiversity. This study addresses a current scientific gap related to the relative contributions of vegetated and unvegetated habitats to biodiversity in tropical regions.

Dissolved primary production and the strength of phytoplankton- bacterioplankton coupling in contrasting marine regions.

We analyzed the strength of phytoplankton-bacterioplankton coupling by comparing the rate of particulate (PPP) and dissolved primary production (DPP) with bacterial carbon demand (BCD) in four contrasting marine regions: offshore and coastal waters of the Southern Ocean, a coastal area of the NE Atlantic, and a coastal-offshore transect in the NW Mediterranean. We measured bacterial heterotrophic production (BHP) and estimated BCD from a literature model. Average phytoplanktonic percent extracellular release [PER = DPP/(DPP + PPP)] was 18-20% in the Antarctic (offshore and coastal, respectively), 16% in the NW Mediterranean, and 7% in the NE Atlantic. A significant inverse relationship was found between PER and total system productivity with pooled data. On average BHP amounted to <5% of total primary production in all regions. However, the strength of phytoplankton-bacterioplankton coupling, estimated as the potential importance of DPP in meeting BCD, differed greatly in the four regions. DPP was highly correlated to BCD in offshore Antarctic waters and was sufficient to meet BCD. In contrast, BCD exceeded DPP and bore no significant relationship in the remaining regions. The data suggest that a strong dependence of bacteria on algal extracellular production is only expected in open-ocean environments isolated from coastal inputs of DOC.