To understand possible factors controlling transmission of trematode larvae between first and second intermediate hosts we examined the impact of ambient fauna on parasite transmission in a marine intertidal parasite-host association. Cockle hosts (Cerastoderma edule) kept together with selected co-occurring macrozoobenthic species in mesocosms acquired a lower parasite load compared to cockles kept alone, when targeted by cercariae of the trematode Himasthla elongata. The reduction of parasite load in the cockles differed between the 7 macrozoobenthic species tested and was between 35 and 91%. Three different types of reduction could be distinguished: (1) predators (Carcinus maenas, Crangon crangon) actively preying upon cercariae, (2) non-host filter feeders (Crepidula fornicata, Mya arenaria, Crassostrea gigas) filtering cercariae but not becoming infected and (3) alternative hosts (Mytilus edulis, Macoma balthica) becoming infected by the cercariae and thus distracting cercariae from the target hosts. In addition, interference competition may occur in the form of disturbance of cockles by ambient organisms resulting in lower filtration rates and subsequently lower parasite loads. Our results suggest that the species composition and relative abundance of the ambient fauna of parasite-host systems play an important role in controlling trematode transmission rates in benthic marine systems.

The macrobenthic production of an estuarine system was evaluated over a 14-year 12 study period in a seagrass bed and in a sandflat. Over this period, the estuary suffered severe 13 eutrophication and extreme weather events with important impacts on the community, 14impairing system functioning and ultimately the goods and services provided by the estuary 15 (decline in the seagrass bed, decreased community production and/or a boost in the 16 production by opportunist species, such as Hydrobia ulvae). Following the anthropogenic 17 impacts, management measures were introduced which allowed a gradual recovery of the 18 seagrass bed and a new macrobenthic community structure manifested by production 19 increases of slow-growing species, such as Scrobicularia plana and Hediste diversicolor. 20There was a gradual re-orientation of energy into population biomass instead of population 21 density but this was not translated into higher community production, mainly due to the 22 decreased production of opportunist species (H. ulvae and several polychaetes). Several 23 weather extreme events occurred during this post-mitigation phase -floods, heatwaves and 24 droughts, all of which had negative impacts on macrobenthic dynamics and production. The 25 33

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