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First recorded in the 1970s, massive green macroalgal blooms have since become an annual recurrence in Brittany, France. Eutrophication (in particular to anthropogenic nitrogen input) has been identified as the main factor controlling Ulva ‘green tide’ events. In this study, we modelled Ulva proliferation using a two-dimensional model by coupling hydrodynamic and biological models (coined ‘MARS-Ulves’) for five sites along the Brittany coastline (La Fresnaye Bay, Saint-Brieuc Bay, Lannion Bay, Guissény Bay and Douarnenez Bay). Calibration of the biological model was mainly based on the seasonal variation of the maximum nitrogen uptake rate (VmaxN) and the half-saturation constant for nitrogen (KN) to reproduce the internal nutrient quotas measured in situ for each site. In each bay, model predictions were in agreement with observed algal coverage converted into biomass. A numerical tracking method was implemented to identify the contribution of the rivers that empty into the study bays, and scenarios of decreases in nitrate concentration in rivers were simulated. Results from numerical nitrogen tracking highlighted the main nitrogen sources of green tides and also showed that each river contributes locally to green tides. In addition, dynamic modelling showed that the nitrate concentrations in rivers must be limited to between 5 and 15 mg l- 1, depending on the bay, to reduce Ulva biomass by half on the coasts. The three-step methodology developed in this study (analysing total dissolved inorganic nitrogen flux from rivers, tracking nitrogen sources in Ulva and developing scenarios for reducing nitrogen) provides qualitative and quantitative guidelines for stakeholders to define specific nitrogen reduction targets for better environmental management of water quality.

We monitored the concentrations of copper, lead and cadmium in seawater, in suspended particulatematter (SPM) and in bacteria, phyto- and zooplankton communities separated from abiogenic particles,over a one year cycle in two coupled Mediterranean coastal ecosystems (Little Bay (LiB) and Large Bay(LaB)). Metals were present in seawater in the order Cu > Pb > Cd in both bays and showed importantvariations within the same month than among months. In LiB, their concentrations were between 0.62and 2.82 l gCul 1 , 0.16 and 19 l gPbl and 0.007 and 0.14 l gCdl 1 , respectively, whereas in LaB, theywere between 0.23 and 2.11 l gCul 1 , 0.09 and 0.76 l gPbl 1 and not detected and 0.65 l gCdl 1 . SPMplay an important role on metal adsorption, especially for copper. Bioaccumulation factors showed thatbacteria and phytoplankton accumulate metals whereas zooplankton tends to biodiminish them in theplankton food web. 2008 Elsevier Ltd. All rights reserved. 1. IntroductionBays are situated between terrestrial and pelagic environmentsand constitute ecotones that may be strongly affected by humanactivities, directly or indirectly, with consequences for the func-tioning of these ecosystems. In these ecosystems, and particularlythose in the Mediterranean Sea, heavy metal concentrations maybe especially high (Danovaro, 2003). Toulon Bay (France, NW Med-iterranean Sea) is a study site of interest for field experiments forseveral reasons. First, it is divided into two systems, Little Bay(LiB) and Large Bay (LaB). These two environments are closelylinked and subject to the same climatic conditions. Second, themorphometrical characteristics and anthropogenic inputs are dif-ferent. Little Bay is a semi-enclosed system, affected by humanactivities including maritime traffic and substantial industrialactivity.Conversely, Large Bay is open to the sea andis less affectedby anthropogenic inputs and, therefore, may be considered ecolog-ically different from Little Bay.Bacteria, phytoplankton and zooplankton are determinants formetal transfer in marine food webs (Michaels and Flegal, 1990),and trace metals are included in plankton biochemical cycles(Whitfield, 2001). Among the heavy metals, copper, lead and cad-mium are of particular interest for different reasons, and may havesignificant effects on the various trophic levels in aquatic foodwebs(Fisher and Reinfelder, 1995). Copper is an essential micronu-trient for phytoplankton communities. It is involved in photosyn-thesis and in iron pathways, essential for cell growth (Peerset al., 2005). However, copper may have a negative effect on bacte-ria (Kim, 1985) on phytoplankton (Sunda and Guillard, 1976) andon the production and the survival of marine copepods (Sundaet al., 1987). Lead and cadmium are both toxic. However, leadcan be toxic at low concentrations, whereas cadmium acts as amicronutrient at low concentrations and becomes toxic at higherconcentrations (Cordero et al., 2005). Suspended particulate matteralso has an important role in the control of metal bioavailability(Sadiq, 1992). There have been several ecotoxicological studies,conducted in the laboratory and as enclosed field experiments; inparticular, these studies have investigated the various pathwaysby which plankton integrate heavy metals. In contrast, few studieshave used natural samples and one reason for this is the difficultyin obtaining pure plankton samples (Ho et al., 2007).Studies addressing how these metals are distributed in littoralplankton communities and which are conducted in situ are rare.We therefore conducted an ecological field study investigatingchanges in heavy metal concentrations in various aspects of LiBand LaB:(1) the changes in concentrations of copper, lead and cadmiumin seawater