Nutrient budget

Environmental requirements and sensitivity of biota

Fate of nutrients

More information on the sensitivity of features of conservation interest to changes in water quality parameters, particularly nutrient concentrations is required. This should be information which could be extrapolated between lagoons of different types but with similar biological communities.

Scientific investigation into the optimum nutrient requirements of Zostera and Ruppia would be useful. Both appear to require moderate nutrient enrichment in order to thrive, but suffer from excessive competition and shading from algae in conditions with excessive nutrients. Little information has been found during the present study on what constitutes excessive nutrients for seagrasses and tasselweeds, or which nutrients may be crucial in this respect. Furthermore, much of the literature on seagrass and nutrients relates to seagrass beds on sand or muddy sand habitats, which tend to be naturally poor in nutrients (at least in terms of retention), whereas in most lagoons where they occur seagrass beds are associated with muddy habitats.

A limited amount of information is available on nutrient requirements of stonewort Lamprothamnium papulosum, and the species is subject to on-going research; depending on the results of such research further detail may be required, e.g. autoecological aspects such as oospore viability and germination conditions.

Controlled laboratory studies could contribute to an understanding of these issues. However, there is a risk in such studies of confounding effects as a result of laboratory, versus natural, conditions. Where appropriate field studies could be designed, it is suggested that the Fleet would provide a suitable site for undertaking autoecological studies in support of the above.

Further study of cycling of nutrients in lagoons between sediments, vegetation and the water column may be very important in terms of ascertaining the bioavailability of nutrients, and assessing the likely effectiveness of any management measures introduced to control nutrients in lagoons. This information should be able to be extrapolated to other lagoon sites with similar sediment characteristics.

Lagoons may often be systems with relatively high nutrient concentrations, but too little is known about what is natural/normal in terms of nutrients in lagoons. Baseline and monitoring studies of lagoonal SACs will assist in building up an information base on water quality in lagoons. Periodic reviews of such data from all lagoons in the UK (and elsewhere) would assist in formulating monitoring and management plans for individual lagoon sites.

One line of investigation not used in the Fleet study which may be appropriate is that of hindcasting (see Johnes et al 1994 and Scott et al 1999). The approach is based on catchment nutrient export modelling and involves determining the relationship between human activities and nutrient inputs from the catchment and then using historical information on changes in human activity, such as land use, to determine past inputs. Such an approach could be followed on the Fleet, building on work already undertaken for the case study.

Another approach might be to take contemporary measurements from a site largely free of anthropogenic impact as an indication of background (surrogate for "natural") levels. In the case study of the Fleet it is concluded that it would be difficult to find a comparable "reference" site; this is likely to be a limitation for many sites.

Depending on the feasibility of using lagoonal sediments, based for example on the results of the research on L. papulosum and attempts to derive a diatom-phosphorus model (Martin 1999), there may be merit in undertaking palaeoenvironmental studies of lagoon sites (hindcasting the biological response) to understand better the natural nutrient characteristics of lagoons. The diatom-phosphorus model will almost certainly require further development to provide more robust phosphorus reconstructions by increasing the number of sites used in the training set (Carvalho pers. comm.).

The hydrodynamic properties of systems such as the Fleet have not been taken account of in numerical hydraulic models. More extensive research is required in the future into studying the effects of these complex hydrodynamic properties, particularly as it relates significantly to the transport of fluid mass and solute contaminant and water quality indicator fluxes in tidal wetlands including large lagoons.

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