Nitrogen

Phosphorus

Distribution and fate of nutrients

Bioavailability

The preceding studies, including mathematical modelling of nutrient budgets and diffuse inputs to the Fleet, indicate the following:

For nitrogen inputs to the Fleet, the principal sources appear to be freshwater streams. Of the seven streams which discharge to the Fleet, the table below gives an indication of their relative annual contributions of nitrogen to the Fleet budget, compared to the estimated contribution from wildfowl.

Nitrogen inputs from streams are greatest during winter, with the source of nitrogen being principally from agricultural fertiliser use. The principal area for arable land cultivation in the Fleet catchment is around Langton Herring, between Roddon and West Fleet streams. The linked figure indicates that all the stream inputs discharge into the Fleet west of Chickerell. However, tilled land is known to be concentrated around the village of Langton Herring and to the east, so it is likely that overall nitrogen loads to the eastern Fleet can be expected to be somewhat higher than those to the western Fleet (Mainstone & Parr 1999).

The Fleet from Chickerell westwards has been shown by the physical modelling exercise to have very poor flushing characteristics.

Estimated nitrogen contribution of sources entering the Fleet (based on

Murdoch 1999)

For phosphorus inputs to the Fleet the situation is less clear. The table below shows the estimated annual contribution of phosphorus to the Fleet from the stream sources and wildfowl inputs. In contrast to the situation for nitrogen, the relative contributions of the sewage treatment works at Abbotsbury and the wildfowl are more important on an annual basis for phosphorus. It should be noted that the estimated input for wildfowl assumes a worst case of all wildfowl faeces entering the lagoon directly rather than only a third with two thirds being deposited on land (as estimated in Mainstone and Parr 1999). If the latter were the case, then the stream inputs (largely from run-off and sewage) would be likely to account for a greater proportion of the nitrogen and phosphorous budgets.

The principal source of phosphorus inputs into the freshwater streams is also estimated to be from agricultural sources, primarily livestock farming (concentrated in the western Fleet), but also from mixed fertiliser application and land runoff as well.

Estimated phosphorus contribution of sources entering the Fleet (based on Murdoch 1999)

However, for phosphorus, in contrast to the situation for nitrogen, there are peaks in loads in both summer and winter. In summer, the principal source of phosphorus appears to be from wildfowl- related inputs but the Abbotsbury STW may also be contributing. The main sources of the winter peaks appear to be freshwater stream inputs, with greatest contributions from Mill stream (Horsepool) and Mill stream (Abbey Barn) at approximately 8 kg/day, but also contributions from Cowards Lake, Rodden stream and West Fleet at approximately 1-2 kg/day. Overall, phosphorus loads from agriculture (mostly dairy farming in the west Fleet) and the STW=s (particularly the larger works at Abbotsbury) are both important, and both occur in the western Fleet (Mainstone & Parr 1999). Again, all these inputs discharge into the poorly flushed part of the Fleet west of Chickerell.

Within the Fleet lagoon itself, there is a west-east trend in nutrients. This trend is due to:

• Higher overall nutrient loadings to the west Fleet than to the east, i.e. higher inputs of phosphorus and also potentially significant inputs of nitrogen (although the east Fleet receives relatively higher inputs of nitrogen);
• Greater flushing (and dilution) of the east Fleet. On average the western Fleet has residence times varying between 10 days during high runoff conditions and 40 days during drought, whereas the eastern Fleet is flushed clear over a few tidal cycles (Robinson 1983); see Section 4.6 for further assessment of flushing times. The weak tidal flushing over the western Fleet basin would result in ponding of freshwater inputs and associated dissolved and suspended substances.

Data obtained during surveys clearly shows freshwater influence waning from Abbotsbury eastwards and that nutrient concentrations decrease with increasing salinity when all the sites are plotted out (EA 1998a), but this is a mere correlation. Salinity might be of relevance as an indicator of which nutrient is limiting plant growth, but this is no longer considered to the case, i.e. nitrogen limitation is just more frequently associated with marine waters (Carvalho, pers. comm.). A more useful indicator is the ratio between nitrates and phosphates. The table below gives estimates of the N:P ratio based on the survey data (linked table), using dissolved available nutrients. All the figures fall close to the critical ratio of between 15:1 and 10:1. Therefore it is uncertain which nutrient would theoretically limit growth first except perhaps at Smallmouth where it is more likely that nitrate is limiting. It is suggested that both nutrients may limit plant growth to varying degrees at different times of the year. However, in practice, nutrients are unlikely to be limiting in the Abbotsbury embayment at present because concentrations are excessively high throughout the year; it can only be postulated as to whether this would be the case under natural conditions.

Estimated N:P ratios at sites within the Fleet using dissolved available nutrients. N is derived from TON plus ammonia. (Based on means from EA nutrient data for April 1996-August 1997.)

In terms of bioavailability of nutrients, and their possible effects on the lagoon in terms of potential for eutrophication, the timing and location of nutrient inputs, as well as their relative size, is vitally important. Inputs entering the Fleet at the western end during the spring and summer are potentially significant due to reduced dilution in summer because of lower rainfall in summer combined with the poor seawater flushing of the western end of the Fleet compared with the east, and the fact that spring/summer is the main plant and algal growing season. Inputs from agriculture may be considerable, but the main periods for nutrient inputs from such sources are in winter during periods of higher rainfall when agricultural ground may be bare and crops are not using up nutrients. However, such nutrients, particularly phosphorus, are likely to become incorporated within the lagoon sediments during winter and taken up by plant growth during spring and summer. It should be noted that whilst most algae (including benthic species as well as plankton and epiphytes) receive all of the nutrients from the water column, most other macrophytes, including charophytes, also utilise sediment sources (Carvalho pers. comm.).

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