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The potential link between the microbial dynamics and the environmental parameters was investigated in a semi-enclosed and highly dynamic coastal system ( Gulf of Trieste, northern Adriatic Sea, NE Mediterranean Sea). Our comprehensive 2-year time-series study showed that despite the shallowness of this area, there was a significant difference between the surface and the bottom bacterial community structure. The bottom bacterial community was more diverse than the surface one and influenced by sediment re-suspension. The surface seawater temperature had a profound effect on bacterial productivity, while the bacterial community structure was more affected by freshwater-borne nutrients and phytoplankton blooms. Phytoplankton blooms caused an increase of G ammaproteobacteria ( A lteromonadaceae, SAR86 and V ibrionaceae) and shift in dominance from SAR11 to R hodobacteraceae taxon at the surface. Our results propose the importance of the water mass movements as drivers of freshwater-borne nutrients and of allochthonous microbial taxa. This study emphasizes the prediction power based on association networks analyses that are fed with long-term measurements of microbial and environmental parameters. These interaction maps offer valuable insights into the response of marine ecosystem to climate- and anthropogenic-driven stressors. [ABSTRACT FROM AUTHOR]

Abstract: The coastal northern Adriatic Sea receives pulsed inputs of riverine nutrients, causing phytoplankton blooms and seasonally sustained dissolved organic carbon (DOC) accumulation—hypothesized to cause episodes of massive mucilage. The underlying mechanisms regulating P and C cycles and their coupling are unclear. Extensive biogeochemical parameters, processes and community composition were measured in a 64-day mesocosms deployed off Piran, Slovenia. We followed the temporal trends of C and P fluxes in P-enriched (P+) and unenriched (P−) mesocosms. An intense diatom bloom developed then crashed; however, substantial primary production was maintained throughout, supported by tightly coupled P regeneration by bacteria and phytoplankton. Results provide novel insights on post-bloom C and P dynamics and mechanisms. 1) Post-bloom DOC accumulation to 186μM remained elevated despite high bacterial carbon demand. Presumably, a large part of DOC accumulated due to the bacterial ectohydrolytic processing of primary productivity that adventitiously generated slow-to-degrade DOC; 2) bacteria heavily colonized post-bloom diatom aggregates, rendering them microscale hotspots of P regeneration due to locally intense bacterial ectohydrolase activities; 3) Pi turnover was rapid thus suggesting high P flux through the DOP pool (dissolved organic phosphorus) turnover; 4) Alpha- and Gamma-proteobacteria dominated the bacterial communities despite great differences of C and P pools and fluxes in both mesocosms. However, minor taxa showed dramatic changes in community compositions. Major OTUs were presumably generalists adapted to diverse productivity regimes.We suggest that variation in bacterial ectohydrolase activities on aggregates, regulating the rates of POM→DOM transition as well as dissolved polymer hydrolysis, could become a bottleneck in P regeneration. This could be another regulatory step, in addition to APase, in the microbial regulation of P cycle and the coupling between C and P cycles. [Copyright &y& Elsevier]

Highlights • Low coherence for GES definitions in Mediterranean plankton communities was found. • Phytoplankton is a key element for assessment systems of MSFD and Regional Conventions. • GES targets are based only on Chlorophyll a thresholds in the Mediterranean so far. • Quantitative GES targets for zooplankton in Mediterranean exist only in Slovenia. • Gaps mostly focus on lack of thresholds and baselines for many biodiversity indicators. Abstract At present there is no consistent approach for the definition of Good Environmental Status (GES) and targets in the Mediterranean Sea, especially for Biodiversity Descriptors, according to the Article 12 of the Marine Strategy Framework Directive (MSFD). The use of plankton indicators in the Mediterranean Sea refers mostly to pelagic habitats in coastal waters and to case studies connected with environmental pressures, e.g. in the Adriatic, Aegean etc. The aim of this review is to study the existing biodiversity indicators for different plankton groups in order to compare GES definitions for the Biodiversity Descriptor and identify the relevant gaps and priority needs to improve coherence for the MSFD implementation across the Mediterranean. For these purposes, we focus on plankton indicators for phytoplankton, zooplankton and prokaryotes. Regional conventions (OSPAR, HELCOM, Barcelona and Bucharest Conventions) have long considered phytoplankton as a key element for integrated assessment systems. Phytoplankton biomass, community composition, abundance, frequency and intensity of blooms are used for such assessment purposes. Chlorophyll a still remains the most widely used indicator mostly thanks to its time saving, cost-effective and reproducible analytical methods that provide easily comparable datasets. Despite some integrated indices proposed for phytoplankton in the literature at the Mediterranean level, a number of constrains still prevent their wide use. Regarding zooplankton communities, commonly used indicators have a taxonomic base while recently size structure and biomass can provide a valuable index of zooplankton population dynamics and ecosystem production. Jellyfish blooms' occurrence and frequency are also considered important zooplankton indicators in specific areas, e.g. North Adriatic. Concerning the prokaryotes, so far MSFD takes into account only their pathogenic component. The revision of MSFD GES definitions shows that all Mediterranean MSs have defined GES at the Descriptor level (e.g. D1 Biodiversity), but our comparison of approaches shows a low level of coherence in GES related to pelagic habitats and plankton communities. Gaps mostly focus on the lack of thresholds and baselines for many biodiversity indicators, and on the scarcity of common and consistent methodological approaches for biodiversity assessment by the MSs. Suggestions to fill these gaps and inconsistencies among MSs include: integration of EU legislation and Regional Agreements and Conventions; targeting on priority species and habitats; testing of existing biodiversity indices with good performances in case studies; coordination and intercalibration actions for the establishment of threshold values and baselines; determination of common methodologies; undertaking of regular monitoring programs and impact assessment studies at regional and sub-regional levels. [ABSTRACT FROM AUTHOR]

This proof of concept study presents a Bayesian Network (BN) approach that integrates relevant biological and physical-chemical variables across spatial (two water layers) and temporal scales to identify the main contributing microbial mechanisms regulating POC accumulation in the northern Adriatic Sea. Three scenario tests (diatom, nanoflagellate and dinoflagellate blooms) using the BN predicted diatom blooms to produce high chlorophyll a at the water surface while nanoflagellate blooms were predicted to occur also at lower depths (>5 m) in the water column and to produce lower chlorophyll a concentrations. A sensitivity analysis using all available data identified the variables with the greatest influence on POC accumulation being the enzymes, which highlights the importance of microbial community interactions. However, the incorporation of experimental and field data changed the sensitivity of the model nodes ≥25% in the BN and therefore, is an important consideration when combining manipulated data sets in data limited conditions. [ABSTRACT FROM AUTHOR]