Microbial plankton communities in the coastal southeastern Black Sea: Biomass, composition and trophic interactions

We investigated biomass and composition of the pico-, nano- and microplankton communities in a coastal station of the southeastern Black Sea during 2011. We also examined trophic interactions within these communities from size-fractionated dilution experiments in February, June and December. Autotrophic and heterotrophic biomasses showed similar seasonal trends, with a peak in June, but heterotrophs dominated throughout the year. Autotrophic biomass was mainly comprised by nanoflagellates and diatoms in the first half of the year, and by dinoflagellates and Synechococcus spp. in the second half. Heterotrophic biomass was mostly dominated by heterotrophic bacteria, followed by nanoflagellates and microzooplankton. Dilution experiments suggest that nano- and microzooplankton were significant consumers of autotrophs and heterotrophic bacteria. More than 100% of bacterial production was consumed by grazers in all experiments, while 46%, 21% and 30% of daily primary production were consumed in February, June and December, respectively. In February, autotrophs were the main carbon source, but in December, it was heterotrophic bacteria. An intermediate situation was observed in June, with similar carbon flows from autotrophs and heterotrophic bacteria. Size-fraction dilution experiments suggested that heterotrophic nanoflagellates are an important link between the high heterotrophic bacterial biomass and microzooplankton. In summary, these results indicate that nano- and microzooplankton were responsible for comprising a significant fraction of total microbial plankton biomass, standing stocks, growth and grazing processes. This suggests that in 2011, the microbial food web was an important compartment of the planktonic food web in the coastal southeastern Black Sea

Distribution, sedimentation and fate of pigment biomarkers following thermal stratification in the western Alboran Sea

A spring investigation of the phytoplankton in the western Alboran Sea (Mediterranean) was undertaken using chlorophyll and carotenoid biomarkers to characterize the community in the water column and in drifting sediment traps set at 100 and 200 m. During 2 drifter experiments, calm and sunny conditions induced a progressive thermal stratification that reduced pigment sedimentation into deeper water and confined the phytoplankton to the surface layer, resulting in an increase in chlorophyll biomass. 19'-Hexanoyloxyfucoxanthin (prymnesiophytes) and chlorophyll b (chlorophytes, prasinophytes, prochlorophytes) were the major accessory pigments, while fucoxanthin, alloxanthin and peridinin indicated the presence of diatoms, cryptophytes and dinoflagellates, respectively. The proportional contribution of each algal group to the chlorophyll a (chl a) biomass, as derived from multiple regression analysis, revealed that prymnesiophytes, cryptophytes and the green algal group collectively accounted for at least 75% in the upper 100 m, emphasizing the importance of the nanophytoplankton. Phaeopigments, dominated by phaeophorbide a2, were the main pigments observed in sediment traps, although chl a, fucoxanthin and 19'-hexanoyloxyfucoxanthin were detected in smaller concentrations as well as traces of chlorophyll b (chl b). In deep water, fucoxanthin and 19'-hexanoyloxyfucoxanthin were the only accessory pigments present while total phaeopigment/chl a molar ratios >1 reflected the active transformation of fine phytogenic material at depth. High particulate organic carbon (POC)/chl a ratios (>100 in surface water; >1000 in deep water) suggested that phytoplankton was a relatively small component of the total carbon biomass down the water column. Using simple budget calculations, we determined that 58 to 65% of the chl a produced in the upper 100 m accumulated in the water column over both experiments. During Expt 1, 29% of the chl a sedimented out, mostly as phaeopigment, at 100 m (24%), and 6% was degraded to colourless residues in the water column. In contrast, only 12% of the chl a sedimented in Expt 2, while 20% was degraded to colourless residues

Reprint of: High prey-predator size ratios and unselective feeding in copepods: A seasonal comparison of five species with contrasting feeding modes

There has been an upsurge of interest in trait-based approaches to zooplankton, modelling the seasonal changes in the feeding modes of zooplankton in relation to phytoplankton traits such as size or motility. We examined this link at two English Channel plankton monitoring sites south of Plymouth (L4 and E1). At L4 there was a general transition from diatoms in spring to motile microplankton in summer and autumn, but this was not mirrored in the succession of copepod feeding traits; for example the ambushing Oithona similis dominated during the spring diatom bloom. At nearby E1 we measured seasonality of food and grazers, finding strong variation between 2014 and 2015 but overall low mesozooplankton biomass (median 4.5 mg C m−3). We also made a seasonal grazing study of five copepods with contrasting feeding modes (Calanus helgolandicus, Centropages typicus, Acartia clausi, Pseudocalanus elongatus and Oithona similis), counting the larger prey items from the natural seston. All species of copepod fed on all food types and differences between their diets were only subtle; the overriding driver of diet was the composition of the prey field. Even the smaller copepods fed on copepod nauplii at significant rates, supporting previous suggestions of the importance of intra-guild predation. All copepods, including O. similis, were capable of tackling extremely long (>500 µm) diatom chains at clearance rates comparable to those on ciliates. Maximum observed prey:predator length ratios ranged from 0.12 (C. helgolandicus) up to 0.52 (O. similis). Unselective feeding behaviour and the ability to remove highly elongated cells have implications for how copepod feeding is represented in ecological and biogeochemical models

Steeper size spectra with decreasing phytoplankton biomass indicate strong trophic amplification and future fish declines

Under climate change, model ensembles suggest that declines in phytoplankton biomass amplify into greater reductions at higher trophic levels, with serious implications for fisheries and carbon storage. However, the extent and mechanisms of this trophic amplification vary greatly among models, and validation is problematic. In situ size spectra offer a novel alternative, comparing biomass of small and larger organisms to quantify the net efficiency of energy transfer through natural food webs that are already challenged with multiple climate change stressors. Our global compilation of pelagic size spectrum slopes supports trophic amplification empirically, independently from model simulations. Thus, even a modest (16%) decline in phytoplankton this century would magnify into a 38% decline in supportable biomass of fish within the intensively-fished mid-latitude ocean. We also show that this amplification stems not from thermal controls on consumers, but mainly from temperature or nutrient controls that structure the phytoplankton baseline of the food web. The lack of evidence for direct thermal effects on size structure contrasts with most current thinking, based often on more acute stress experiments or shorter-timescale responses. Our synthesis of size spectra integrates these short-term dynamics, revealing the net efficiency of food webs acclimating and adapting to climatic stressors

Costs and benefits to European shipping of ballast-water and hull-fouling treatment: Impacts of native and non-indigenous species

Maritime transport and shipping are impacted negatively by biofouling, which can result in increased fuel consumption. Thus, costs for fouling reduction can be considered an investment to reduce fuel consumption. Anti-fouling measures also reduce the rate of introduction of non-indigenous species (NIS). Further mitigation measures to reduce the transport of NIS within ballast water and sediments impose additional costs. The estimated operational cost of NIS mitigation measures may represent between 1.6% and 4% of the annual operational cost for a ship operating on European seas, with the higher proportional costs in small ships. However, fouling by NIS may affect fuel consumption more than fouling by native species due to differences in species’ life-history traits and their resistance to antifouling coatings and pollution. Therefore, it is possible that the cost of NIS mitigation measures could be smaller than the cost from higher fuel consumption arising from fouling by NIS

Low salinity as a biosecurity tool for minimizing biofouling on ship sea chests

Biofouling is a major vector in the transfer of non-native species around the world. Species can be transported on virtually all submerged areas of ships (e.g. hulls, sea chests, propellers) and so antifouling systems are used to reduce fouling. However, with increased regulation of biocides used in antifoulants (e.g. the International Maritime Organization tributyltin ban in 2008), there is a need to find efficient and sustainable alternatives. Here, we tested the hypothesis that short doses of low salinity water could be used to kill fouling species in sea chests. Settlement panels were suspended at 1.5 m depth in a Plymouth marina for 24 months by which time they had developed mature biofouling assemblages. We exposed these panels to three different salinities (7, 20 and 33) for 2 hours using a model sea chest placed in the marina and flushed with freshwater. Fouling organism diversity and abundance were assessed before panels were treated, immediately after treatment, and then 1 week and 1 month later. Some native ascidian Dendrodoa grossularia survived, but all other macrobenthos were killed by the salinity 7 treatment after 1 week. The salinity 20 treatment was not effective at killing the majority of fouling organisms. On the basis of these results, we propose that sea chests be flushed with freshwater for at least 2 hours before ships leave port. This would not cause unnecessary delays or costs and could be a major step forward in improving biosecurity

Reconciliation of the carbon budget in the ocean’s twilight zone

Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean1, 2. The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage3. It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude4, 5, 6, 7, 8. Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink

Blue pigmentation of neustonic copepods benefits exploitation of a prey-rich niche at the air-sea boundary

The sea-surface microlayer (SML) at the air-sea interface is a distinct, under-studied habitat compared to the subsurface and copepods, important components of ocean food webs, have developed key adaptations to exploit this niche. By using automated SML sampling, high-throughput sequencing and unmanned aerial vehicles, we report on the distribution and abundance of pontellid copepods in relation to the unique biophysicochemical signature of the SML. We found copepods in the SML even during high exposure to sun-derived ultraviolet radiation and their abundance was significantly correlated to increased algal biomass. We additionally investigated the significance of the pontellids’ blue pigmentation and found that the reflectance peak of the blue pigment matched the water-leaving spectral radiance of the ocean surface. This feature could reduce high visibility at the air-sea boundary and potentially provide camouflage of copepods from their predators

Grazing by Calanus helgolandicus and Para-Pseudocalanus spp. on phytoplankton and protozooplankton during the spring bloom in the Celtic Sea

Feeding rates and selectivity of the calanoid copepods Calanus helgolandicus and Para-Pseudocalanus spp. on natural assemblages of microplankton were evaluated in the English Channel and western Celtic Sea during non-bloom and bloom conditions in April 2002. Ingestion rates of total chlorophyll-a were low at non-bloom stations where the phytoplankton community was dominated by cells < 5 ?m in length and higher during the bloom when the > 5 ?m size fraction was dominant. Protozooplankton contributed to the copepod diet in all experiments, C. helgolandicus clearance and ingestion rates were highest for the ciliate Myrionecta rubra (626–1347 ml cop? 1 d? 1; 0.3–27 ?g C cop? 1 d? 1). C. helgolandicus ingested between 1 and 18 ?g C cop? 1 d? 1 (1–12% body C) from phytoplankton + protozooplankton food sources. The total carbon ingested by Para-Pseudocalanus spp. was lower (0.5–6 ?g cop? 1 d? 1) but this was equivalent to between 6 and 78% of body carbon being ingested daily. Our data suggest that C. helgolandicus selected prey according to size; this was not the case for Para-Pseudocalanus spp. which became more selective as chlorophyll-a concentration increased. Grazing impact of the entire copepod community on protozooplankton was assessed. We found that at non-bloom stations between 12 and 17% of the protozooplankton community was being removed daily by the copepod community, whereas during the peak of the bloom the proportion being removed daily was only 2%. We conclude that during the spring bloom period copepods gained the majority of their carbon from phytoplankton ingestion but during non-bloom periods, protozooplankton and the ciliate M. rubra made a significant contribution to copepod diet