English Channel From WGRED 2008 From WGRED 2008

1.2.3.2.1 Phytoplankton and Zoolplankton

Phytoplankton

Among the identified microalgae in the Eastern Channel are diatoms (genus Nitzschia, Thalassiosira, Rhizosolenia, Chaetoceros, Skeletonema and Gyrodinium, Gymnodinium, Ceratium for dinoflagelates). For the areas of the Seine and Somme estuaries, diverse marine , brackish and fresh water population of diatoms are mixing some being planktonic and others benthic. In the Channel, some toxic and harmful species form blooms with direct effect on marine animals (eg: Dictyocha speculum, Prorocentrum minimum and P. micans, Gymnodinium cf nagasakiense) and human consumption (Dinophysis and Alexandrium) (Belin and

Martin-Jézéquel, 1997). The spatial distribution of the primary production may vary largely seasonally but is often concentrated along the coasts (Figure 1.2.52).

Figure 1.2.52: Chlorophyll a concentration in May 2007 from satellite images (calculated after Gohin et al., 2002, source http://www.ifremer.fr/cersat/facilities/browse/del/roses/browse.htm).

Zooplankton

In the Eastern Channel, the stability of the hydrological structure orientated along the coast gives rise to a specific coastal ecosystem slowly drifting northward with a characteristic coastal and offshore assemblage of plankton. These two ecosytems are separated by a relatively narrow front (Brylinski and Lagadeuc, 1990).

The most represented groups are the copepods, Sagitta setosa (chaetognathe), cladocera and jellyfish as well as numerous benthic invertebrate larvae (Cirripedia,

Annelidae and Echinodermae). In the western Channel, the zooplankton is subject to the influence of Atlantic water mainly in areas deeper than 50 m. Fauna is more diversified than in the eastern part of the Channel and an offshore (central western channel) assemblage may be distinguished from a coastal assemblage (<30 m) (Le Fèvre-Lehoërff et al., 1997).

communities (Castel et al., 1997). Along this gradient, associated benthic assemblages often follow the same repartition (Cabioch, 1968; Gentil, 1976; Retière, 1979) (Figure 1.2.53).

Figure 1.2.53: Macrobenthic community distribution in the Eastern English Channel (modified by Carpentier et al., 2005 after Sanvicente Anorve, 2002).

In the western Channel, due to its particular tidal conditions, intertidal and subtidal zones characterised by a large variety of habitats occur. This translates into a very large faunistic and floristic species richness.

1.2.3.2.3 Fish communities

Most European commercial species are present in the Channel. Of the 100 (or so) species that contribute to the catches, about 40 species constitute 90% of the landed biomass. It is unusual to have so many species contributing to the bulk of the commercial landings, This is possibly due to the relative shallowness and large variety of habitats in the area, but also it may be related to the structure of the local fishery which supports a large number of small coastal vessels

exploiting diversified resources to meet a varied demand in consumption with less reliance on large pelagic fisheries as experienced elsewhere. Large life history traits (benthic, demersal, pelagic) and taxonomic diversity may be noted as flat fish (sole, plaice,…), gadoids (cod, whiting, hake…), elasmobranches (skates, sharks, dogfish), crustaceans (crabs, spider crab, lobster), cephalopods (squids and cuttlefish), shellfish (scallops, whelk) and algae may all be found in the area (Guitton et al., 2003). Some species are considered as resident as particularly attached to some Channel biotopes (scallops, whelk, algae), others are seasonal, following their migration (mackerel) or reproductive cycle (herring, seabass, cuttlefish). Most are species with a larger geographic distribution that may be found indiscriminately in the Channel or in adjacent areas.

(sole, whiting,… ), some displaying some Atlantic preference (hake, squids, anglerfish) or rather some North Sea attachment (Cod, herring). Numerous spawning and nursery grounds as well as migratory routes of many species occur in the Channel.

Populations are distributed along the main ecological gradients resulting in a combination of cold and temperate water species, the Channel being the thermal partition limit for some of them (southern limit for cod and whiting, and northern limit for hake and anglerfish).

The spatial distribution of some species has been recently described over the last two decades in the Eastern English Channel (Carpentier et al., 2005, http://charm.canterbury.ac.uk/). This distribution was strongly structured by the local abiotic environment most of which have remained stable over this period. Studies of the fish, cephalopod and macro-invertebrate assemblages in this area have identified four distinct community types that are determined by environmental factors such as depth, salinity, water temperature, seabed shear stress, and sediment type (Figure 1.2.54). From 1997 to 2004, some 25% of overall community structure variance could be related to the available environmental descriptors and 20% to persistent factors such as depth, seabed shear stress, sediment, and macro-invertebrate community type.

The different communities differ in their species diversity and are highest in areas of soft

Figure 1.2.54: Spatial distribution of fish sub-communities in the Eastern Channel from 1988 to 2004.

Gradation from open sea community to coastal and estuarine communities is shown (Vaz et al., 2007).

Rochet et al. (2005) assessed two estuarine communities (Seine and Somme estuaries) identified as nursery areas for commercially important stocks exploited elsewhere in mixed fisheries. In both estuaries, no significant evolution trends were found. They also concluded that overall the fish stock populations of the Eastern Channel were not deteriorating.

This result was also supported by recent studies showing that although the Eastern Channel fish communities displayed significant inter-annual variation in both structure and composition over the last two decades, the different communities, and their spatial distribution, are persistent over time reflecting the relative stability of environmental conditions in the area (Vaz et al., 2007). Overall, however, species diversity over the entire region appears to have increased over the last two decades, (Vaz et al., 2007). The figures produces in this paper were updated by the authors and are presented below.

Figure 1.2.55: Species richness per haul (the line represents the average) The species richness evolution from 1988 to 2007 is significant (p<0.001).

rich coastal assemblages well adapted to changing temperature and salinity conditions. This pattern is correlated to an overall increase in species richness at the level of the haul translating an increase in the species co-existence (Figure 1.2.55). This may be induced by higher spatial heterogeneity and more variable conditions.

1.2.3.2.4 OTHER ISSUES - KNOWLEDGE GAPS

Primary and secondary production data in the Channel originate mainly from very coastal areas.

These data are too restricted temporally and spatially to be extrapolated to the whole region (Lacroix et al., 2007). Also primary production and large functional group of phytoplankton have been modeled with success in the Channel (Menesguen et al., 2007), these models still require field data or satellite imagery to be better calibrated. Secondary production patterns are still not well described.

The sediment and benthic invertebrates distribution are over 30 years old and these data certainly need updating. A better understanding of the overall effect of aggregate extraction activities over the whole system (sediment dynamic, coastline erosion, benthic invertebrates, fish) is also required to anticipate possible adverse effect of mineral resources exploitation in the Channel.

There is gaps in the knowledge of fish distribution and abundance evolution in the Western Channel. CEFAS historic and future surveys may enable to close these gaps in the future.

Synthetic information of seabirds and marine mammals are available but often aggregated to adjacent regions. These need to be added to account for upper trophic levels.

Major significant ecological events and trends

Hawkins et al. (2003) reviewed the changes in marine life abundance recorded in the Western Channel off Plymouth and related them to environmental change and human activity. They have shown that, from the 1920s to the 1950s, there was a period of warming of the sea, with

increases in abundance of species of fish, plankton and intertidal organisms that are typically common in warmer waters to the south of Britain. This period was followed by a cooler period where northern cold-water species became more abundant but over-exploitation prevented them to return to abundance levels close to those observed at the beginning of the century.

Since the 1980s regional sea-surface temperature has warmed again and abundances of warm-water species are increasing.

Some warm-water species, in particular red mullet and common squid, exhibited a strong evolution in their abundance over the last few years. The following figures are indices computed by IFREMER (Franck.Coppin@ifremer.fr) from the Channel Ground Fish Survey observed

abundance in October in the Eastern Channel onboard the Ifremer RV “Gwen Drez”. These indices include all age classes and may translate strong recruitment (Figure 1.2.56 and Figure 1.2.57). In both cases, a new fishery is developing.

Figure 1.2.56: Red Mullet (Mullus surmulletus) average density (nb/km²) with 95% confidence interval.

1.2.3.3 Pelagic