Thermaikos Gulf is a eutrophic area located in the Northwestern part of the Aegean Sea in the Eastern Mediterranean. Interspecific differences among various filter feeders in their ability to ...accumulate okadaic acid, were observed during natural blooms of
Dinophysis acuminata in the gulf. Okadaic acid analyses by high performance liquid chromatography (HPLC) were performed on benthic specimens and
D. acuminata cell densities and cell toxin content were estimated in water samples. Seven filter feeding species were collected in the gulf during two DSP outbreaks in May 2003 and March 2004. The various species showed a different potential to accumulate okadaic acid in their tissues. The highest concentrations were found in the mussel populations (
Mytilus galloprovincialis and
Modiolus barbatus), while among the non-bivalve filter feeders, ascidians were the main accumulators of okadaic acid. The rest of shellfish populations (
Flexopecten proteus,
Chlamys varia and
Venus verrucosa) were found to contain toxins only during 2004, when
D. acuminata densities were found above 10000
cells
l
−1.
M. galloprovincialis was proved to be the most appropriate indicator for a safe warning of okadaic acid contamination in Thermaikos Gulf.
Abundance, biomass, vertical distribution, species composition and mixotrophy of planktonic ciliates were investigated during March 1997 in the oligotrophic Eastern Mediterranean. Six depth layers ...were sampled in the euphotic zone (0 to 100 m) at 7 stations in the North and 4 in the South Aegean Sea, resulting in a total of 66 samples and 82 ciliate species. Abundance was in general very low (0 to 780 cells 1-1) except from specific stations in the North Aegean stations (20 to 2040 cells 1-1), influenced by the Black Sea Water. Aloricate species dominated the depth-integrated abundance and biomass in both the North and the South. South Aegean stations presented more diverse ciliate fauna, uniform distribution down to 100 m and higher values of integrated abundance and biomass than the North Aegean stations. Among North stations, those subjected to the influence of the Black Sea presented very pronounced stratification, with mixotrophs and heterotrophs decreasing sharply below 20 m; at these stations, mixotrophic species of 18 to 30 μm dominated the ciliate community at the surface whereas at the South as well as at the rest of the North stations, nanociliates were the major mixotrophic size class. Mixotrophs (20% of abundance in the South, 38% in the North) and nanociliates (28% of abundance in the South, 44% in the North) were found to be very important components of the community, which indicates that these organisms play also an important role in oligotrophic systems.
Dynamics of autotrophic and heterotrophic prokaryotes and their consumption by nanoflagellates were studied in the euphotic zone at nine stations located from the Levantine Basin (34°E) to the ...Balearic sea (5°E) in June 1999. Bacterial biomass constituted the largest proportion of living biomass at all stations. Integrated bacterial production at the furthest east station, was sixfold lower than integrated bacterial production at the furthest west (13 and 75 mg C m−2 d−1 respectively). Estimated heterotrophic nanoflagellate bacterivory accounted for 45–87% of bacterial production. Small protists (<3 μm) dominated the bacterivore assemblage and accounted for more than 90% of the heterotrophic bacterial consumption. Our results indicated that there was no negative selection against Synechococcus and that both picoplankton groups were grazed according to their standing stocks. An estimated consumption of Synechococcus derived from food vacuole content analysis of nanoflagellates revealed that they consumed from 0.5 to 45% (mean 13%) of Synechococcus stock per day. These data are among the first documenting the relative grazing impact of heterotrophic nanoflagellates on bacteria and Synechococcus in situ. Assuming that there was no selection for or against Prochlorococcus, heterotrophic nanoflagellates could ingest from 1.4 to 21% (mean 6%) of Prochlorococcus stock per day. The amount of organic carbon obtained by heterotrophic nanoflagellates from photosynthetic prokaryotes represented 27% of the total amount of carbon obtained from total prokaryotes
The effects of atmospheric deposition on plankton community structure wereexamined during a mesocosm experiment using water from the Cretan Sea (EasternMediterranean), an area with a high frequency ...of atmospheric aerosol deposition events.The experiment was carried out under spring-summer conditions (May 2012). Themain objective was to study the changes induced from a single deposition event,on the autotrophic and heterotrophic surface microbial populations, from viruses tozooplankton. To this end, the effects of Saharan dust addition were compared to theeffects ofmixed aerosol deposition on the plankton community over 9 days. The effects ofthe dust addition seemed to propagate throughout the food-web, with changes observedin nearly all of the measured parameters up to copepods. The dust input stimulatedincreased productivity, both bacterial and primary. Picoplankton, both autotrophic andheterotrophic capitalized on the changes in nutrient availability and microzooplanktonabundance also increased due to increased availability of prey. Five days after thesimulated deposition, copepods also responded, with an increase in egg production.The results suggest that nutrients were transported up the food web through autotrophs,which were favored by the Nitrogen supplied through both treatments. Although, theeffects of individual events are generally short lived, increased deposition frequencyand magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in planktoncommunity structure. Here we demonstrate how a single dust deposition event leadsto enhancement of phytoplankton and microzooplankton and can eventually, throughcopepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.
Abundance of benthic bacteria, heterotrophic nanoflagellates and ciliates, extracellular enzymatic activities, bacterial C production, C mineralisation and sediment community oxygen consumption rates ...were measured in the Thermaikos Gulf (Northeastern Mediterranean), before (September 2001), and during intense trawling activities (October 2001 and February 2002). The biochemical composition of sedimentary organic matter has revealed that bottom trawling had an effect on the trophic state of Thermaikos Gulf. Changes on the benthic microbial food web were also recorded, during the three sampling seasons. Even though trawling-induced sediment resuspension did not alter significantly the abundance of the microbial components, with the exception of the most impacted station, it determined changes regarding their relative importance. Thus, the ratios of bacterium to nanoflagellates and ciliate to nanoflagellates abundance increased in the trawled stations, causing a sudden increase in bacterial C production, in comparison to the non-trawled station. Four months later, the effects of trawling on the microbial food web were less evident, masked possibly by the drastic decrease in the water temperature. The results of the present work suggest that bottom trawling induces alteration of the sedimentological variables and can be considered as a factor affecting the function of the microbial food web in marine coastal ecosystems. These alterations cause faster mobilisation of organic C buried in the sediment and increase nutrient concentrations and availability in the system, thus inducing an effect that could lead to coastal eutrophication.
We examined and quantified the contributions of copepods and appendicularians to the vertical flux of carbon during autumn and spring in Gullmar Fjord (west coast of Sweden). Faecal pellet-production ...rate was determined for major copepod and appendicularian species. In addition, house-production rates were estimated for the appendicularianOikopleura dioica. Vertical flux of pigments, faecal carbon and appendicularian houses were measured using short-term (24 h) deployments of sediment traps at 2 depths (15 and 30 m). Copepods dominated the community biomass in both spring and autumn and their pellets dominated the faecal carbon flux.O. dioicahouses with attached detritus were an important component of the biogenic carbon flux in October (15.3 mg C m–2d–1), equalling the contribution from copepods at 15 m and 50% of the flux at 30 m. At that time, we observed a loss rate of 70% d–1of the houses produced in the water column. In the spring, althoughFritillaria borealisdominated the appendicularians, its houses did not appear to contribute to the biogenic flux. Our results suggest that oikopleurids and fritillariids may not operate equivalently in biogeochemical cycles. Because of the significant contribution of appendicularians to carbon fluxes, they should be incorporated in future flow models of coastal oceans
Copepods and appendicularians are major grazers in the pelagic environment. They have different retention efficiencies for prey and may therefore exert a variable grazing pressure on the spectrum of ...pico- to micro-plankton. We determined clearance rates of both groups at one station during 24 h in the Gullmar fjord, west Sweden, in autumn 1999. Total potential prey biomass ranged from 75 µg C l
−1
at the surface to 14 µg C l
−1
at 30 m with a dominance of larger dinoflagellates (10-25 µm athecate species and Gymnodinium /Gyrodinium sp.) and the pennate diatom Pseudo-nitzschia sp. Grazer biomass was dominated by copepods (Acartia clausi, Paracalanus parvus) and appendicularians (Oikopleura dioica). O. dioica showed non-selective clearance rates of 0.7-1.8 ml µg C
−1
h
−1
on most diatoms, flagellates and ciliates, whereas Pseudo-nitzschia sp. and dinoflagellates and ciliates >25 µm were not removed by O. dioica. Appendicularian grazing impact was 0.06% d
−1
on the phytoplankton and 0.4% d
−1
on bacterial biomass. Despite a seven-fold higher biomass, the grazing impact of copepods on phytoplankton biomass was only 0.28% d
−1
indicating that O. dioica had a proportionally greater impact and, in contrast to copepods, also utilised bacteria. The low observed grazing impact was due to a low grazer biomass and a prey community largely unavailable to the investigated grazers.
A mesocosm experiment was conducted in order to studythe structure of the planktonic food web. The dynamicsof pico-, nano- and microplankton populations werefollowed during 40 days in four large (40 ...m^sup 3^)enclosures. In three tanks a gradient of addednutrients (nitrogen and phosphorus) was applied, whilea fourth tank was used as a control. On day 14, thetop predator (sea bream Sparus aurata larvae)was introduced into the tanks and part of the watercolumn in each tank was isolated in a plastic bagwithout fish larvae, to act as a control forpredation. Physical parameters, chlorophyll aand nutrient concentrations, as well as planktonconcentrations were monitored. A diatom bloom wasobserved in all four tanks, in the first phase endingwith silicate depletion. Flagellate and dinoflagellateabundance subsequently increased, these organismsbeing limited by zooplankton grazing. The zooplanktonpopulations were controlled by both resources (mostlyflagellates) and predation (by fish larvae) asindicated by the results of the control experiments.PUBLICATION ABSTRACT
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