Consequences of nitrate enrichment in a temperate estuarine marine protected area; response of the microbial primary producers and consequences for management
Poole Harbour is a large temperate estuary and Marine Protected Area (MPA). It is also a major site of bivalve aquaculture and is subject to rising nutrient concentrations. Dissolved inorganic nitrogen concentrations are predicted to rise in the MPA over, at least, the next 20 years mainly due to historical fertiliser applications in the catchment and therefore nitrogen enrichment will continue to modify the Poole Harbour ecosystem. Nitrate enrichment clearly has consequences for the ecology of the harbour and therefore has implications for the objectives of the MPA and the way in which the MPA should be managed. Currently, the ‘primary’ consequence of nitrate enrichment is thought to be the proliferation and accumulation of green macroalgae such as Ulva due to the very high affinities of this taxon for nitrate. The consequences of nitrate enrichment for the harbour microalgae are less clear. Using new and existing datasets, we assess the response of the microbial primary producers to the ongoing nitrogen enrichment and highlight two principal features of the datasets: the high abundances of the smallest phytoplankton (picoeukaryote algae) as well as the very high abundances of nanoeukaryote algae (mostly chain-forming diatoms) which occur every summer, to a varying or lesser degree. Picoeukaryote algae have recently been recognised as a potentially significant component in the nutrition of some bivalves which may therefore contribute significantly to the growth of commercially-important bivalves in the harbour. Finally, we discuss the research needed to better understand the potential interactions between a potentially expanded shellfish aquaculture industry in Poole Harbour and the desired ecological functioning of the Poole Harbour MPA.