Dominique Jamet

The zooplankton community in bays of Toulon area (northwest Mediterranean Sea, France)

The abundance and the diversity of the zooplankton community were studied during an annual cycle in three shallow bays differently affected by anthropogenic inputs of Toulon area in the northwest Mediterranean Sea (France). Environmental variables of water quality were also considered. Our results discriminated Little Bay from the La Garonne and Niel Bay. Little Bay, heavely perturbed by anthropogenic inputs, had high chlorophyll a levels, abundant zooplankton, a low diversity index, a high dominance Index and a single dominant species (Oithona nana). La Garonne Bay and Niel Bay, much less polluted, had low chlorophyll a levels, low zooplankton abundance, low dominance Index and high diversity index and eveness. In addition, O. nana occurred rarely in the latter bays. Zooplankton successions were also different between Little Bay and the two other bays.

Alkaline phosphatase in a littoral Mediterranean marine ecosystem: role of the main plankton size classes

Abstract This work investigates the alkaline phosphatase activity in a littoral marine ecosystem (Toulon Bay and Le Niel Bay, France) in order to study its biochemical characteristics with respect to pH, sea water composition and phosphate sensitivity. We also characterise the active forms in sea water and determine the extent to which zooplankton generate phosphatase activity with respect to other plankton classes. In Toulon Bay, phosphatase was produced mostly by the microplankton fraction (>90 μm), accounting for more than 90% of total activity. In contrast, most of the phosphatase activity in Le Niel Bay was generated by the nanoplankton fraction (5–90 μm) and the picoplankton fraction (0.25–5 μm). The microplankton enzymes had non Michaelis-Menten kinetics suggesting the involvement of multiple enzyme processes with distinct kinetic constants. This activity is in major part secreted into the sea water and is stimulated by the ionic strength and the pH of the sea water. Cypris larvae of the genus Balanus played a special role in this release. For the nanoplankton and picoplankton, part of this activity was due to non-secreted enzymes, probably bound to membranes or occurring intracellularly. Moreover, nano and picoplankton phosphatase required higher pH than microplankton enzyme. For all plankton size classes, there was no activity at low pH, suggesting that acid phosphatases were not involved in reactions with substrates dissolved in water.

Comprehensive Model of Annual Plankton Succession Based on the Whole-Plankton Time Series Approach

Ecological succession provides a widely accepted description of seasonal changes in phytoplankton and mesozooplankton assemblages in the natural environment, but concurrent changes in smaller (i.e. microbes) and larger (i.e. macroplankton) organisms are not included in the model because plankton ranging from bacteria to jellies are seldom sampled and analyzed simultaneously. Here we studied, for the first time in the aquatic literature, the succession of marine plankton in the whole-plankton assemblage that spanned 5 orders of magnitude in size from microbes to macroplankton predators (not including fish or fish larvae, for which no consistent data were available). Samples were collected in the northwestern Mediterranean Sea (Bay of Villefranche) weekly during 10 months. Simultaneously collected samples were analyzed by flow cytometry, inverse microscopy, FlowCam, and ZooScan. The whole-plankton assemblage underwent sharp reorganizations that corresponded to bottom-up events of vertical mixing in the water-column, and its development was top-down controlled by large gelatinous filter feeders and predators. Based on the results provided by our novel whole-plankton assemblage approach, we propose a new comprehensive conceptual model of the annual plankton succession (i.e. whole plankton model) characterized by both stepwise stacking of four broad trophic communities from early spring through summer, which is a new concept, and progressive replacement of ecological plankton categories within the different trophic communities, as recognised traditionally.

Role of sea water DIP and DOP in controlling bulk alkaline phosphatase activity in N.W. Mediterranean Sea (Toulon, France)

The regulation of alkaline phosphatase activity by dissolved inorganic (DIP) and organic phosphorus (DOP) and the contribution of DOP as phosphorus source were studied monthly in Toulon Bay (NW Mediterranean, France) in 2005-2006. The concentrations of DIP and DOP varied respectively from 0 to 0.185μM and from 0 to 0.329μM. The bulk activities (Vm, Km, Vm/Km) were measured using MUFP as substrate. Its high affinity component (Km: 0.05-1.00μM) was negatively correlated with the sum of the concentrations of DIP and DOP but not with these compounds taken independently. A negative correlation with DIP was found when the concentrations of DOP were lower than 0.08μM. A negative correlation with DOP was shown when the concentrations of DIP were lower than 0.05μM. This high affinity component can be considered as a valuable indicator for the potential utilization of the compounds which contribute to the intracellular phosphorus pool.

Characterisation of marine zooplankton alkaline phosphatase activity in relation to water quality

In this study, we measured the non-specific zooplanktonic phosphate activity inside and outside Toulon bay. These activities were compared in relation to substrate and phosphate concentrations, pH and sea water composition. Great differences were observed between the two sites. Phosphatase activities were higher inside Toulon bay, where high zooplankton density and high protein concentrations were observed. Concerning enzyme activities, the intersite differences increased at the lowest substrate concentrations. Moreover, zooplankton alkaline phosphatase activity showed a good adaptation to the natural medium conditions (pHs and ionic strength) and is likely directly liberated to the water, where it contributes to an enrichment in mineral phosphorus from organic material.

Seasonal changes in zooplanktonic alkaline phosphatase activity in Toulon Bay (France): the role of Cypris larvae.

We studied zooplankton contribution to the total particulate phosphatase activity, the kinetics of this activity, the relation to the different taxonomic groups and the role of particle-bound bacteria. The activity of total particulate material collected from a liter of seawater was more elevated in May, June and August than during the rest of the year. These high activities resulted from a high contribution of the >90 microm fraction which account then for more than 60% of the total particulate activity. Two Michaelian processes with high and low V(max) were disclosed on this fraction. The high V(max) component was responsible for the high summer activities. During these periods, high densities of cirriped Cypris were found which were statistically correlated with this high V(max) component as with its specific activity. Moreover, the contribution of attached bacteria to these high activities was low. In return, this contribution was predominant during the periods of low activity. A simple method was developed to characterise this bacterial activity.

Seawater DMS in a perturbed coastal ecosystem

DMS concentrations, chlorophyll a concentrations, abiotic parameters of water quality and quantitative samples of plankton were carried out once a month from January to December 1997 into two zones of a semi-enclosed french littoral ecosystem (Toulon Bay, NW Mediterranean sea). This bay is divided into two subecosystems by an artificial breakwater: the inner bay (polluted zone, P) is largely influenced by anthropogenic perturbations and the outer bay (less polluted, LP) is much less polluted. We found greater concentrations of DMS and chlorophyll a, of phytoplankton and zooplankton densities and biomasses in the polluted zone (P) than in the less polluted zone (LP) of the bay. The DMS concentration and phytoplankton biomass were strongly correlated, and a high degree of eutrophication may contribute, in connection with other factors, to a greater production of phytoplankton which in turn enhances the DMS production. The DMS concentration in coastal polluted zones is then greatly higher than in open sea (around three times) and this greater production should be taken into account for the global estimation, at least on a local scale, of DMS production in seawater, which is a key factor for the biogenic sulfur cycle.

The relationships between particulate and soluble alkaline phosphatase activities and the concentration of phosphorus dissolved in the seawater of Toulon Bay (NW Mediterranean)

The activities of particulate and soluble phosphatase were analyzed monthly for 1 year in the coastal ecosystems of the NW Mediterranean Sea. The mean contribution of the particulate activity increased from 56% at an MUF-P concentration of 30 μM to 77% at 0.04 μM. This particulate activity was negatively correlated with the DIP, DOP and TDP concentrations when the activities were related to the seawater volume, chlorophyll a or the protein concentration. The TDP correlations were highly significant (p: 0.001). The DOP correlations were significant (p: 0.04) and became highly significant (p: 0.009) at low DIP concentrations (<0.13 μM). The DIP correlations were significant (p: 0.04) only at low DOP concentrations (<0.18 μM). Thus, the effects of seawater DIP and DOP were found to be linked. The soluble activity exhibited distinct phosphatase fractions with high (0.5-29.5 μM) and low (0.02-2 μM) Km values, but none exhibited significant correlations with phosphorus compounds.

Contribution of copepods, cladocerans and cirripeds to phosphatase activity in mediterranean zooplankton

The hydrolysis of dissolved phosphoric esters by zooplankton was studied in Toulon seaport (N.W Mediterranean Sea, France). Zooplankton was collected on 90 μm filters. The enzyme reaction was studied using paranitrophenyl phosphate (p-NPP) dissolved in sea water. The particulate material activity was expressed as Vmax (reaction rate when the enzyme is saturated with substrate) using Eadie-Hofstee plots. The activity per L. was high in spring and decreased from summer to winter. Biomass and zooplankton abundance varied also widely during the year. There were significant correlations between activity per L. and the cirriped cyprid abundance. In contrast, during periods when cyprids were absent, positive correlations with copepod and cladoceran abundance were found. Specific activity varied also during the year, with high values in spring and low values in winter. This activity was positively correlated with cyprid abundance, but not with total zooplankton, copepod or cladoceran abundance. These results indicated that copepods and cladocerans contributed to the overall enzyme production, and that cirripeds could be responsible of high specific activities.

Role of DOP on the alkaline phosphatase activity of size fractionated plankton in coastal waters in the NW Mediterranean Sea (Toulon Bay, France)

The particulate material was fractionated into 5 size classes (>90μ, 50-90μ, 6-50μ, 1-6μ, and <1μ). DOP was analysed as easily (DOPh, DOPpa) and less easily hydrolysable compounds (DOPox). Based on Vmax, 94% of the high affinity AP activity was due to <50μ cells and 77% to <1μ cells. 83% of the low affinity activity was due to >90μ cells. The high affinity activities were negatively correlated with DOP for the <50μ classes. These correlations came mostly from DOPox. They were more significant when NO3+NO2 concentrations were high, when DIP concentrations were low and when N/P ratio was >10. At lower N/P ratios, AP was more significantly correlated with DIP. The low affinity activities showed significant negative correlation with DIP and with DOP and DOPox for the >90μ class. The inhibition of AP activities by DOPox may originate from stable compounds interfering with DIP for the control of AP synthesis.