Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). II. Investigating the effects of high trophic levels on nutrients and plankton dynamics and associated feedbacks

The end-to-end OSMOSE-GoL model parameterized, calibrated and evaluated for the Gulf of Lions ecosystem (Northwestern Mediterranean Sea) has been used to investigate the effects of introducing two-ways coupling between the dynamics of Low and High Trophic Level groups. The use of a fully dynamic two-ways coupling between the models of Low and High Trophic Levels organisms provided some insights in the functioning of the food web in the Gulf of Lions. On the whole microphytoplankton and mesozooplankton were found to be preyed upon by High Trophic Levels planktivorous groups at rates lower than 20% and 30% of their respective natural mortality rates, but these relatively low rates involved some important alterations in the infra-seasonal and annual cycles of both High and Low Trophic Levels groups. They induced significant changes in biomass, fisheries landings and food web interactions by cascading effects. Spatial differential impacts of High Trophic Levels predation on plankton are less clear except in areas in which primary productivity is high. Higher predation rates on plankton groups were encountered within the area of the Rhone river's influence and in areas associated to the presence of mesoscale eddies in the Northwestern part of the Gulf of Lions, especially. Generally, the pressure of the High Trophic Levels predation was the highest in areas of highest biomass whatever the plankton group considered. The two-ways coupling between Low and High Trophic Levels models revealed both bottom-up and top-down controls in the ecosystem with effects on planktivorous species similar to those observed in the field. The use of the end-to-end model enabled to propose a set of potential mechanisms that may explain the observed decrease in small pelagic catches by the French Mediterranean artisanal fisheries over the last decade

Seasonal variation of stable isotope ratios of size-fractionated zooplankton in the Bay of Marseille (NW Mediterranean Sea)

Stable isotope ratios of six size fractions of zooplankton (80 to >2000 mm) were analyzed seasonally in 2009-2010 at the SOMLIT site in the Bay of Marseille. Isotopic ratios generally increased with zooplankton size. The highest delta N-15 values were observed in the 1000-2000 mm fraction. The largest size class (>2000 mu m), dominated by gelatinous plankton, had lower delta N-15 values due to the low isotopic signatures of most of these organisms. In the larger size fractions (>1000 mu m), isotopic ratios were measured at the taxon level. Brachyuran, stomatopod, teleost and cephalopod larvae showed the highest delta N-15 values, and salps and pteropods the lowest ones. Lower values of both delta C-13 and delta N-15 were recorded in winter and spring than in summer and autumn for all fractions. Seasonal variations were consistent with the fluctuations of environmental parameters (temperature, nutrients, Chl a concentration) and were related to phytoplankton and seawater particulate organic matter (POM) composition. Stable isotope and flow cytometry analysis of water POM indicated that sewage wastewater particles were mixed with marine phytoplankton at the SOMLIT site and transferred up into the zooplanktonic food web

Computed MM5 winds (10 m above sea level) at Somlit station.

<p>MM5 wind intensities and directions, clockwise from the north in degrees, computed (3 km resolution) every 3 hours at 10 m above sea level at a location corresponding to Somlit station; a) forcing conditions for the computation of June 2008; b) forcing conditions for the computation of October 2011; times for particle release (WWTP outlet) and model outputs (current fields and particle trajectories) are mentioned in each time series; Note the different scale of wind vectors in Figs 3a and 3b.</p

Spatial distribution of the WWTP initial patch of particles.

<p>The amount of particles reaching each coastal site is expressed in terms of fractions (%) of the initial patch of 5.10⁴ particles, during the computations from June 15, 0:00 to July 7, 0:00, 2008 and from October 14, 6:00 to October 20, 0:00, 2011.</p

Modelling results of the starting intrusion (June 2008 and October 2011).

<p>(a) and (c): fields of surface current (arrows) and elevation (ζ) computed by MARS3D at the sea surface on June 17, 0:00, 2008, and October 15, 0:00, 2011, respectively. (b) and (d): trajectories of the passive particles computed by ICHTHYOP, relating to the same period as the corresponding current fields, on June 17, 6:00, 2008, and October 15, 0:00, 2011, respectively, over the 1–2 days following the particle release at the WWTP outlet; colors refer to the depths (m) reached by each particle <i>black</i> near the surface (< 10 m depth), <i>yellow</i> sub-surface (10–30 m depth) and <i>red</i> at depth (> 30 m depth).</p

Stable isotope ratios of the surface seawater POM.

<p>Sampling at Somlit on October 17, 2011; comparison with ratios of the main sources of POM identified in the bays of Marseille; the 2 main sources of POM are indicated by arrows.</p

Time series of particle amounts reaching the southern bay.

<p>Computation from June 15, 0:00, to July 7, 0:00, 2008; areas impacted: SE Frioul, Récifs, Prophète, Prado and Madrague; <i>red arrows</i> refer to the time lapse since the particle release.</p

Wind rose measured at the Frioul meteorological station (altitude: 74 m).

<p>Frequency distribution of directions from which hourly averaged winds with speeds > 2 m^<b>.</b>s^-1 originated during 1976–1998, measured clockwise from north in degrees.</p

Conceptual diagram of connectivity from the WWTP outlet.

<p>(a): diagram showing the start of particle transport in the vicinity of the Cortiou WWTP outlet, following 2 paths according to E-SE (wind 1) and S-SE (wind 2) wind conditions. (b): comparative diagram of particle connectivity from the Cortiou WWTP outlet with and without intrusion of the NC on the shelf and associated with E-SE and S-SE winds; <i>grey path</i>: particle transport at the surface induced by SE winds without NC intrusion; <i>solid black path</i>: particle transport at the surface induced by NC intrusion and E-SE wind; <i>dotted black path</i>: particle transport near the surface (< 10 m depth) induced by NC intrusion and E-SE wind; <i>red path</i>: particle transport sub-surface (about 20 m depth) induced by NC intrusion and S-SE wind; <i>thick black arrow</i> refers to the downwelling at the southeastern coastline of the Frioul archipelago; <i>thick red arrow</i> refers to the downwelling front of the WWTP outlet; <i>grey dotted line</i> refers to the presumed northwestern limit of the WWTP impact zone.</p

Time series of particle amounts reaching the western limit and northern bay.

<p>Computation from June 15, 0:00 to July 7, 0:00, 2008; areas impacted: Somlit, NW Frioul, Rouet, Niolon and Joliette; <i>red arrows</i> refer to the time lapse since the particle release.</p