Morphology, fluid Motion and Predation by the Scyphomedusa Aurelia Aurita

Although medusan predators play demonstrably important roles in a variety of marine ecosystems, the mechanics of prey capture and, hence, prey selection, have remained poorly defined. A review of the literature describing the commonly studied medusa Aurelia aurita (Linnaeus 1758) reveals no distinct patterns of prey selectivity and suggests that A. aurita is a generalist and feeds unselectively upon available zooplankton. We examined the mechanics of prey capture by A. aurita using video methods to record body and fluid motions. Medusae were collected between February and June in 1990 and 1991 from Woods Hole, Massachusetts and Narragansett Bay, Rhode Island, USA. Tentaculate A. aurita create fluid motions during swimming which entrain prey and bring them into contact with tentacles. We suggest that this mechanism dominates prey selection by A. aurita. In this case, we predict that medusae of a specific diameter will positively select prey with escape speeds slower than the flow velocities at their bell margins. Negatively selected prey escape faster than the medusan flow velocity draws them to capture surfaces. Faster prey will be captured by larger medusac because flow field velocity is a function of bell diameter. On the basis of prey escape velocities and flow field velocities of A. aurita with diameters of 0.8 to 7.1 cm, we predict that A. aurita will select zooplankton such as barnacle nauplii and some slow swimming hydromedusae, while faster copepods will be negatively selected

A global database of dissolved organic matter (DOM) concentration measurements in coastal waters (CoastDOM v1)

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) con-centrations are used to characterize the dissolved organic matter (DOM) pool and are important components ofbiogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOMv1; available at https://doi.org/10.1594/PANGAEA.964012, L\uf8nborg et al., 2023) compiling previously pub-lished and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complementedby hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables(e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon andtotal alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However,most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the SouthernHemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC,20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequenceDOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focuson carbon cycling by the aquatic science community. The global database shows that the average DOC concen-tration in coastal waters (average \ub1 standard deviation (SD): 182 \ub1 314 μmol C L−1; median: 103 μmol C L−1) is13-fold higher than the average coastal DON concentration (13.6 \ub1 30.4 μmol N L−1; median: 8.0 μmol N L−1),which is itself 39-fold higher than the average coastal DOP concentration (0.34 \ub1 1.11 μmol P L−1; median:0.18 μmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and willhelp facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochem-ical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing abaseline for modelling future changes in coastal waters

Planktonic ciliate distribution relative to a deep chlorophyll maximum: Catalan sea, NW Mediterranean, June 1993

Vertical distributions and relative contributions of distinct trophic guilds of ciliates were investigated in an oligotrophic system with a deep chlorophyll maximum (DCM) in early summer. Ciliates were classified as helerotrophic: micro and nano ciliates, tintinnids and predacious forms or photosynthetic: large mixotrophic oligotrichs (Laboea strobilia, Tontonia spp.), and the autotrophic Mesodinium rubrum. Variability between vertical profiles (0-200 m) was relatively low with station to station differences (C.V.s of similar to 30%) generally larger than temporal (1-4 day) differences (C.V.s of similar to 15%), for integrated concentrations. Total ciliate biomass, based on volume estimates integrated from 0-80 m, averaged similar to 125 mg C m(-2), compared to similar to 35 mg m(-2) for chlorophyll a (chi a), yielding a ciliate to chi ratio of 3.6, well within the range of 1 to 6 reported for the euphotic zones of most oceanic systems. Heterotrophic ciliate concentrations were correlated with chi a concentration (r = 0.83 and 0.82, biomass and cells l(-1), respectively) and averaged similar to 230 cells l(-1) in near surface samples (chi a = 0.1 mu g l(-1)) to similar to 850 cells l(-1) at 50 m depth, coinciding with the DCM (chi a = 1-2 mu g l(-1)). Tintinnid ciliates were diverse (36 species in 19 general but a minor part of heterotrophic ciliates. Nanociliates represented <1% of heterotrophic or total ciliate biomass, in contrast to reports on near-shore ciliate communities. Predacious ciliates were very rare. Large mixotrophic oligotrichs, while a minor portion of ciliate cells l(-1), were an important part of total ciliate biomass, representing 63% at 5 m and 21% integrated over 0-80 m. Mesodinium rubrum was found throughout the water column, usually with a sub-surface peak (similar to 100 cells l(-1)). Concentrations of neither large mixotrophic oligotrichs, nor the autotrophic M. rubrum, were correlated with chi a. Estimates of the contribution of photosynthetic ciliate chi (mixotrophic and autotrophic) to total chi a (based on literature values of chi a cell(-1)) ranged from similar to 20% in some surface samples to <0.5% in the DCM

Effects of nutrients and turbulence on the production of transparent exopolymer particles: a mesocosm study

International audienceThe production of transparent exopolymer particles (TEP) in response to several environmental variables was studied in 2 mesocosm experiments. The first (Expt 1) examined a gradient of 4 nutrient levels; the second (Expt 2) examined different conditions of silicate availability and zooplankton presence. Tanks were separated in 2 series, one subjected to turbulence and the other not influenced by turbulence. In tanks with nutrient addition, TEP were rapidly formed, with net apparent production rates closely linked to chl a growth rates, suggesting that phytoplankton cells were actively exuding TEP precursors. High nutrient availability increased the absolute concentration of TEP; however, the relative quantity of TEP produced was found to be lower, as TEP concentration per unit of phytoplankton biomass was inversely related to the initial nitrate dose. In Expt 1, an increase in TEP volume (3 to 48 mu m equivalent spherical diameter) with nutrient dose was observed; in Expt 2, both silicate addition and turbulence enhanced TEP production and favored aggregation to larger TEP (> 48 mu m). The presence of zooplankton lowered TEP concentration and changed the size distribution of TEP, presumably by grazing on TEP or phytoplankton. For lower nutrient concentrations, the ratio of particulate organic carbon (POC) to particulate organic nitrogen (PON) followed the Red-field ratio. At higher nutrient conditions, when nutrients were exhausted during the post-bloom, a decoupling of carbon and nitrogen dynamics occurred and was correlated to TEP formation, with a large flow of carbon channeled toward the TEP pool in turbulent tanks. TEP accounted for an increase in POC concentration of 50% in high-nutrient and turbulent conditions. The study of TEP dynamics is crucial to understanding the biogeochemical response of the aquatic system to forcing variables such as nutrient availability and turbulence intensity

Particulate DNA and protein relative to microorganism biomass and detritus in the Catalano-Balearic Sea (NW Mediterranean) during stratification

Using microscopic and biochemical approaches, the relative contribution of the main groups of pelagic microorganisms (bacteria, heterotrophic nanoflagellates, phytoplankton and ciliates) and detritus (<150 mu m) to total particulate protein and DNA was investigated at two stations of the Catalano-Balearic Sea (NW Mediterranean) during the stratified period. The two stations, one located in the shelf break front (S) and the other in the open sea, above the central divergence zone (D), were sampled twice in early summer 1993. Both of them showed a well-developed deep chlorophyll maximum (DCM). Maximum DNA concentrations were observed close to the DCM, while protein concentrations were fairly homogeneous from the surface to 60 m depth in all samplings. In general, the microorganism distribution showed maximum concentrations at or near the DCM depths. At both stations, bacteria were the most important contributors to living particulate DNA (22.5-32.6%), while phytoplankton and heterotrophic nanoflagellates were the main contributors to living particulate protein (3.8-24.4 and 2.9-29.1%, respectively). In addition, an important amount of detrital DNA and protein was estimated to occur at both stations. Detrital DNA accounted for 23.9-42.9% of the particulate DNA, while detrital protein represented from 63.5 to 84.7% of the particulate protein. Because both protein and DNA contain nitrogen and DNA is also a phosphorus source, these results indicate that heterotrophic organisms and detrital particles play an important role in the nitrogen and phosphorus cycles in the open sea waters of the NW Mediterranean