Abstract
Hypoxia is a major stressor on biological communities in many oceanic and coastal ecosystems. Various size-dependent processes (e.g. growth and reproduction rates, predator–prey ...interactions) are adversely affected by hypoxia. We hypothesized that the impacts of hypoxia on planktonic communities would also be reflected in their Normalized Biomass Size Spectra (NBSS) as steeper slopes and lower intercepts. To explore this hypothesis, we studied the planktonic communities (from bacteria to mesozooplankton) of Elefsina, an enclosed bay that exhibits near bottom hypoxia during summer, and Aghios Kosmas, an adjacent coastal site outside the bay. Bottom layer hypoxia formed during the stratification period in Elefsina Bay significantly altered the distribution of planktonic organisms in the water column. Several unicellular and mesozooplanktonic groups avoided the hypoxic layer, in which the biomass of autotrophic picoeukaryotes was markedly higher. Community changes related to hypoxia were clearly reflected in the NBSS. The slope was significantly steeper in the hypoxic layer (−1.330 vs −1.193) and the intercept was lower (−2.222 vs −0.972, hypoxic vs oxic layer). This result can be interpreted as reduced trophic transfer efficiency to the higher trophic levels due to hypoxia.
Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and ...bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.
In food webs, interactions between competition and defence control the partitioning of limiting resources. As a result, simple models of these interactions contain links between biogeochemistry, ...diversity, food web structure and ecosystem function. Working at hierarchical levels, these mechanisms also produce self‐similarity and therefore suggest how complexity can be generated from repeated application of simple underlying principles. Reviewing theoretical and experimental literature relevant to the marine photic zone, we argue that there is a wide spectrum of phenomena, including single cell activity of prokaryotes, microbial biodiversity at different levels of resolution, ecosystem functioning, regional biogeochemical features and evolution at different timescales; that all can be understood as variations over a common principle, summarised in what has been termed the ‘Killing‐the‐Winner’ (KtW) motif. Considering food webs as assemblages of such motifs may thus allow for a more integrated approach to aquatic microbial ecology.
Climate change driven by human activities encompasses the increase in atmospheric CO
concentration and sea-surface temperature. Little is known regarding the synergistic effects of these phenomena on ...bacterial communities in oligotrophic marine ecosystems that are expected to be particularly vulnerable. Here, we studied bacterial community composition changes based on 16S rRNA sequencing at two fractions (0.1-0.2 and >0.2 μm) during a 10- day fully factorial mesocosm experiment in the eastern Mediterranean where the pH decreased by ~0.3 units and temperature increased by ~3 °C to project possible future changes in surface waters. The bacterial community experienced significant taxonomic differences driven by the combined effect of time and treatment; a community shift one day after the manipulations was noticed, followed by a similar state between all mesocosms at the third day, and mild shifts later on, which were remarkable mainly under sole acidification. The abundance of
increased in response to warming, while the
clade immediately benefited from the combined acidification and warming. The effect of the acidification itself had a more persistent impact on community composition. This study highlights the importance of studying climate change consequences on ecosystem functioning both separately and simultaneously, considering the ambient environmental parameters.
Recent estimates of nutrient budgets for the Eastern Mediterranean Sea (EMS) indicate that atmospheric aerosols play a significant role as suppliers of macro- and micro- nutrients to its Low Nutrient ...Low Chlorophyll water. Here we present the first mesocosm experimental study that examines the overall response of the oligotrophic EMS surface mixed layer (Cretan Sea, May 2012) to two different types of natural aerosol additions, “pure” Saharan dust (SD, 1.6 mg l-1) and mixed aerosols (A - polluted and desert origin, 1 mg l-1). We describe the rationale, the experimental set-up, the chemical characteristics of the ambient water and aerosols and the relative maximal biological impacts that resulted from the added aerosols. The two treatments, run in triplicates (3 m3 each), were compared to control-unamended runs. Leaching of approximately 2.1-2.8 and 2.2-3.7 nmol PO4 and 20-26 and 53-55 nmol NOx was measured per each milligram of SD and A, respectively, representing an addition of approximately 30% of the ambient phosphate concentrations. The nitrate/phosphate ratios added in the A treatment were twice than those added in the SD treatment. Both types of dry aerosols triggered a positive change (25-600% normalized per 1 mg l-1 addition) in most of the rate and state variables that were measured: bacterial abundance (BA), bacterial production (BP), Synechococcus (Syn) abundance, chlorophyll-a (chl-a), primary production (PP) and dinitrogen fixation (N2-fix), with relative changes among them following the sequence BP>PP≈N2-fix>chl-a≈BA≈Syn. Our results show that the ‘polluted’ aerosols triggered a relatively larger biological change compared to the SD amendments (per a similar amount of mass addition), especially regarding BP and PP. We speculate that despite the co-limitation of P and N in the EMS, the additional N released by the A treatment may have triggered the relatively larger response in most of the rate and state variables as compared to SD. An implication of our study is that a warmer atmosphere in the future may increase dust emissions and influence the intensity and length of the already well stratified water column in the EMS and hence the impact of the aerosols as a significant external source of new nutrients.
Plankton community composition changes in response to nutrient enrichment were examined at 2 sites in the eastern Mediterranean. Samples from bacteria up to mesozooplankton were taken upstream and ...downstream of finfish farms in the north Aegean (Lesvos, Greece) and on the southern coast of Cyprus. The pattern of change appeared to be similar, albeit of different magnitude, in both areas. In Cyprus, results showed an increase in bacteria and a decrease in diatom abundance close to the cages, response to effluents was limited, and consequent growth was controlled by microzooplankton. In Lesvos, results showed increased abundance of bacteria, nanoflagellates and mesozooplankton, a decrease in diatoms and an increase in larger dinoflagellates and ciliates downstream of the farm. A shift towards a more diverse microplankton community was observed, consistent with the intermediate disturbance hypothesis. The community consisted of larger-sized microzooplankton, and changes persisted throughout the downstream stations. Results showed that the plankton response to nutrient enrichment is more pronounced at intermediate distances from the farm. The community composition changes observed in Lesvos indicate that even in exposed sites with high current velocities there is an influence of farm cages. Changes manifest in terms of size for some groups and abundance for others. Although fish farms have been associated with degradation of the environment in which they are located, this was not apparent in the present study. Indeed, the shifts induced by farm effluents can help us identify drivers of change and assess community responses to perturbations in a dynamic environment.
The speciation of dissolved iron (DFe) in the ocean is widely
assumed to consist almost exclusively of Fe(III)-ligand complexes. Yet in
most aqueous environments a poorly defined fraction of DFe also ...exists as
Fe(II), the speciation of which is uncertain. Here we deploy flow injection
analysis to measure in situ Fe(II) concentrations during a series
of mesocosm/microcosm/multistressor experiments in coastal environments in
addition to the decay rate of this Fe(II) when moved into the dark. During
five mesocosm/microcosm/multistressor experiments in Svalbard and Patagonia,
where dissolved (0.2 µm) Fe and Fe(II) were quantified
simultaneously, Fe(II) constituted 24 %–65 % of DFe, suggesting that
Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to
decay in the dark, the vast majority of measured oxidation rate constants
were less than calculated constants derived from ambient temperature,
salinity, pH, and dissolved
O2. The oxidation rates of Fe(II) spikes added to Atlantic seawater
more closely matched calculated rate constants. The difference between
observed and theoretical decay rates in Svalbard and Patagonia was most
pronounced at Fe(II) concentrations <2 nM, suggesting that the effect may
have arisen from organic Fe(II) ligands. This apparent enhancement of Fe(II)
stability under post-bloom conditions and the existence of such a high
fraction of DFe as Fe(II) challenge the assumption that DFe speciation in
coastal seawater is dominated by ligand bound-Fe(III) species.
Combining a minimum food web model with Arctic microbial community dynamics, we have suggested that top-down control by copepods can affect the food web down to bacterial consumption of organic ...carbon. Pursuing this hypothesis further, we used the minimum model to design and analyse a mesocosm experiment, studying the effect of high (+Z) and low (-Z) copepod density on resource allocation, along an organic-C addition gradient. In the Arctic, both effects are plausible due to changes in advection patterns (affecting copepods) and meltwater inputs (affecting carbon). The model predicts a trophic cascade from copepods via ciliates to flagellates, which was confirmed experimentally. Auto- and heterotrophic flagellates affect bacterial growth rate and abundance via competition for mineral nutrients and predation, respectively. In +Z, the model predicts low bacterial abundance and activity, and little response to glucose; as opposed to clear glucose consumption effects in -Z. We observed a more resilient bacterial response to high copepods and demonstrate this was due to changes in bacterial community equitability. Species able to use glucose to improve their competitive and/or defensive properties, became predominant. The observed shift from a SAR11-to a Psychromonodaceae - dominated community suggests the latter was pivotal in this modification of ecosystem function. We argue that this group used glucose to improve its defensive or its competitive abilities (or both). Adding such flexibility in bacterial traits to the model, we show how it creates the observed resilience to top-down manipulations observed in our experiment.
The extracellular concentration of H2O2 in surface aquatic environments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to ...removeH2O2 from solution. In any kind of incubation experiment, the formation rates and equilibrium concentrations of reactive oxygen species (ROSs) such as H2O2 may be sensitive to both the experiment design, particularly to the regulation of incident light, and the abundance of different microbial groups, as both cellular H2O2 production and catalase–peroxidase enzyme production rates differ between species. Whilst there are extensive measurements of photochemical H2O2 formation rates and the distribution of H2O2 in the marine environment, it is poorly constrained how different microbial groups affect extracellular H2O2 concentrations, how comparable extracellular H2O2 concentrations within large-scale incubation experiments are to those observed in the surface-mixed layer, and to what extent a mismatch with environmentally relevant concentrations of ROS in incubations could influence biological processes differently to what would be observed in nature. Here we show that both experiment design and bacterial abundance consistently exert control on extracellular H2O2 concentrations across a range of incubation experiments in diverse marine environments.During four large-scale (>1000 L) mesocosm experiments (in Gran Canaria, the Mediterranean, Patagonia and Svalbard) most experimental factors appeared to exert only minor, or no, direct effect on H2O2 concentrations. For example, in three of four experiments where pH was manipulated to 0.4–0.5 below ambient pH, no significant change was evident in extracellular H2O2 concentrations relative to controls. An influence was sometimes inferred from zooplankton density, but not consistently between different incubation experiments, and no change inH2O2 was evident in controlled experiments using different densities of the copepod Calanus finmarchicus grazing on the diatom Skeletonema costatum (<1 % change in H2O2 comparing copepod densities from 1 to 10 L-1). Instead, the changes in H2O2 concentration contrasting high- and low-zooplankton incubations appeared to arise from the resulting changes in bacterial activity. The correlation between bacterial abundance and extracellular H2O2 was stronger in some incubations than others (R2 range 0.09 to 0.55), yet high bacterial densities were consistently associated with low H2O2. Nonetheless, the main control on H2O2 concentrations during incubation experiments relative to those in ambient, unenclosed waters was the regulation of incident light. In an open (lidless) mesocosm experiment in Gran Canaria, H2O2 was persistently elevated (2–6-fold) above ambient concentrations; whereas using closed high-density polyethylene mesocosms in Crete, Svalbard and Patagonia H2O2 within incubations was always reduced (median 10 %–90 %) relative to ambient waters.
Editorial on the research topic Impact of Aerosols (Saharan Dust and Mixed) on the East Mediterranean Oligotrophic Ecosystem, Results from Experimental Studies In oligotrophic environments, dust and ...nutrient inputs via atmospheric deposition are considered important sources of macro-nutrients and micro-trace metals fueling primary and secondary production. ...there are three more papers; one on the effect of dust-associated airborne microbes deposited into the sea on marine autotrophic and heterotrophic production (Rahav et al.), one on the impact of dry deposition on the sea surface microlayer (Astrahan et al.) and a last one describing a long term flux of Saharan dust to the suburban area of Athens, Greece (Vasilatou et al.). ...we would like to acknowledge the contribution of the handling editor as well as of numerous reviewers to the completion of the present special issue.
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