Abstract. Transparent exopolymer particles (TEP) are the most ubiquitous gel particles in the ocean and form abiotically from dissolved precursors. Although these particles can accumulate at the ocean surface, being thus exposed to intense sunlight, the role of solar radiation for the assembly and degradation of TEP is unknown. In this study, we experimentally determined the effects of visible and ultraviolet B (UVB) radiation on (1) TEP degradation (photolysis experiments), (2) TEP assembly from dissolved polymers (photoinhibition experiments) and (3) TEP release by microorganisms. Solar radiation, particularly in the UVB range, caused significant TEP photolysis, with loss rates from 27 to 34% per day. Dissolved polysaccharides did not increase in parallel. No TEP were formed under UVB, visible or dark conditions, indicating that light does not promote TEP assembly. UVB radiation enhanced TEP release by microorganisms, possibly due to cell deaths, or as a protective measure. Increases in UVB may lead to enhanced TEP photolysis in the ocean, with further consequences for TEP dynamics and, ultimately, sea-air gas exchange.
Data on hake (Merluccius capensis and Merluccius paradoxus) abundance along the Namibian coast showed both species to have strongly aggregated spatial distributions. In addition, average fish size increased with depth, suggesting that the formation of hake aggregation is based on size-dependent shoaling behaviour. Hake spacing was size dependent with areal fish density and biomass being scaled to the −5 and −2 power, respectively, of fish length. The equations describing the size dependence of the areal fish density were log density (fish∙m −2 ) = 5.06-4.91 log size (cm) for M. capensis and log density (fish∙m −2 ) = 5.28-4.74 log size (cm) for (M. paradoxus). The maximum areal densities, presumably corresponding to those within the shoals, exceeded those described by the equations above by 20- and 13-fold for M. capensis and M. paradoxus, respectively. The size dependence of hake spacing implies that fishing pressure targeting aggregates of small hake (i.e. shallow shoals) should have a disproportionate effect on fish mortality and recruitment compared with similar fishing pressure targeting aggregates of larger hake (i.e. deep shoals).
The proliferation of a number of pressures affecting the ocean is leading to a growing concern that the state of the ocean is compromised, which is driving society into pessimism. Ocean calamities are disruptive changes to ocean ecosystems that have profound impacts and that are widespread or global in scope. However, scrutiny of ocean calamities to ensure that they can be confidently attributed to human drivers, operate at widespread or global scales, and cause severe disruptions of marine social-ecosystems shows that some of the problems fail to meet these requirements or that the evidence is equivocal. A number of biases internal and external to the scientific community contribute to perpetuating the perception of ocean calamities in the absence of robust evidence. An organized auditing of ocean calamities may deliver a more precise diagnosis of the status of the oceans, which may help to identify the most pressing problems that need be addressed to conserve a healthy ocean.
Net community production (NCP) is a relevant parameter informing on the role of plankton communities as sinks or sources of CO 2 in the ocean. We assessed the effect of ultraviolet (UV) radiation on NCP in different regions along the coast of Western Australia (WA). We compared 57 NCP measurements of surface communities receiving full-spectrum natural solar radiation, to communities with natural UVB radiation excluded (NCP -UV ). Very high values of incident UVB radiation were registered, especially during spring and summer (up to 75 kJ m -2 d -1 ). Although a strong inhibitory effect of UVB on NCP was expected due to the high UVB levels, no significant differences between NCP and NCP -UV were observed in any sampled region. However, we found a general trend of NCP inhibition by 33.4% under UVB radiation along the WA coast. While NCP of autotrophic communities tended to slightly decrease under UVB radiation, no robust pattern was observed for heterotrophic communities. Experiments measuring the response of NCP exposed to a gradient of enhanced UVB doses exhibited a UV dose-dependent inhibition of NCP in estuarine communities (i.e. higher inhibition with higher UVB doses), and a UV dose-independent inhibition in coastal communities. Our in situ data showed that the net metabolism of surface plankton communities from the coast of WA was insignificantly affected by elevated environmental UVB radiation levels.
The role of krill as a source of dissolved organic matter in the Southern Ocean was tested through a series of experiments performed around the Antarctic Peninsula. These experiments revealed high but variable release rates of dissolved material (carbon and nutrients), supplying, on average, 150 mmol dissolved organic carbon (DOC) m −2 d −1 , which is comparable with that supported by phytoplankton. Krill support, on average, 73% of the combined krill + phytoplankton production of DOC in the ecosystem, implying the importance of krill in conditioning the productivity of the Southern Ocean. However, the contribution of krill as a source of DOC varied greatly because of the patchy distribution of both krill and primary producers in the region, ranging from 98% to 10% of the combined (krill + phytoplankton) DOC release rates. These results suggest that rapid decline in krill standing stocks associated with reduced ice cover may have major consequences for microbial communities in the ecosystem, since bacterial carbon demand often exceeds the DOC supplied by phytoplankton in coastal areas of the Southern Ocean, with potential unforeseen consequences in the carbon balance of the Southern Ocean.
