This paper presents data for the temporal and spatial distribution of nutrients in Liverpool Bay between 2003 and 2009 and an analysis of inputs of nutrients from the major rivers. The spatial distribution of winter nutrient concentrations are controlled by the region of freshwater influence (ROFI) in Liverpool Bay through the mixing of riverine freshwater and Irish Sea water, with strong linear relationships between nutrient concentration and salinity between December and February. The location of highest spring and summer phytoplankton biomass reflects the nutrient distributions as controlled by the ROFI. Analysis of 7 years of data showed that the seasonal cycle of winter maximum nutrient concentrations in February and drawdown in April/May is a recurrent feature of this location, with the timing of the drawdown varying by several weeks between years. A comparison of observed nutrient concentrations in Liverpool Bay with those predicted from inputs from rivers has been presented. Nutrient concentrations in the rivers flowing into Liverpool Bay were highly variable and there was reasonable agreement between predicted freshwater nutrient concentrations using data from this study and riverine nutrient concentrations weighted on the basis of river flow, although the exact nature of mixing between the rivers could not be determined. Predicted Irish Sea nutrient concentrations in the winter were lower than those reported for the input waters of the North Atlantic, supporting findings from previous work that nitrogen is lost through denitrification in the Irish Sea.
Complex coastal currents control early-stage larval dispersal from intertidal populations, and late-stage settlement patterns, but are often poorly resolved in larval dispersal models. Generally, there is high uncertainty in the timing of larval spawning, which markedly affects larval dispersal. In this study, we describe the physical parameters that induce spawning events in the blue mussel, Mytilus edulis, using a variation of the Condition Index (which relates the mass of meat to the mass of the shell) as a proxy. We developed a high-resolution Eulerian coastal hydrodynamic model, coupled with a Lagrangian particle tracking model, to quantify the potential dispersal of early-stage mussel larvae based on differing spawning dates obtained from field data. Our results showed that (1) the timings of larval spawning cannot be explained solely by ‘thermal shocks’ in the sea or air temperatures (i.e. fluctuations in temperature causing stress); (2) larger spawning events generally occurred during neap tides; (3) the simulated larval dispersal was largely but not always predicted by averaged current pathways (calculated over two weeks period); and (4) simulated self-recruitment was low at sites associated with strong tidal currents. These results have important implications for shellfisheries stock management and sustainability. Specific to this study, simulated mussels from shellfishery beds off North Wales dispersed more than 25 km in one week and so could feasibly contribute to the wider population throughout the northern part of the Irish Sea.
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