Information from assessing biota

Information from assessing nutrients

There appears to be no reliable historical evidence for a decline in either Zostera, Ruppia, or stonewort Lamprothamnium papulosum populations in the Fleet since the late 1960's. There appear to have been changes in Zostera and Ruppia distributions with time, but these appear to have been natural changes, associated with hard winters and ice damage, or recovery of Zostera from the wasting disease in the 1930's.

With respect to Lamprothamnium papulosum, however, it is possible that it was common in the Abbotsbury embayment during the last century, as samples from the Natural History Museum collected then are labelled as taken from Abbotsbury (L. Carvalho, pers.comm.). However, no published data are available to confirm whether this was the case: publications in the Fleet Study Group archive only indicate its presence along the northern shores of the lagoon further eastwards, where it was reported as thriving in 1991 (Holmes, 1993). Sampling in 1998 failed to find any Lamprothamnium papulosum at Abbotsbury (A.Martin, pers. comm.). If, however, the environmental requirements of Lamprothamnium papulosum are considered, it is likely that the species has been affected at the western end of the Fleet. Studies indicate that the species appears to be absent from potentially suitable sites when levels of soluble reactive phosphate are greater than 30 µg/l-P, and is most frequently found where levels are less than 10 µg/l-P (Martin, 1999). Mean levels of orthophosphate (approximately equivalent to soluble reactive phosphate) at Chickerell Hive, Langton Hive and in the Abbotsbury embayment exceeded 20 µg/l-P from 1996 to 1997, with mean levels below 10 µg/l-P found only at Smallmouth (EA 1998a).

There is little information on lagoonal invertebrate or fish populations in the Fleet, and certainly not enough to determine whether significant changes in their populations have taken place.

There is no reliable evidence either for increases in epiphytic or benthic green algal growth which might affect species of conservation interest. However, the lack of evidence appears primarily due to a lack of quantitative historical information on populations of green algae. The review of historic literature in the FSG archive suggests that lush spring and summer growths of epiphytic and benthic green algae on the Zostera and Ruppia beds in the Fleet, as well as in the Abbotsbury embayment where seagrass is sparse, have occurred certainly since the late 1960's and probably earlier. However, there have been no quantitative surveys to determine whether the species composition of such growths has changed or not, or whether the density of algal growth has increased or decreased over this time.

Furthermore, there is no reliable evidence for increases in phytoplankton growth which might also affect species of conservation interest, and might indicate nutrient enrichment. Again, the lack of evidence appears primarily due to a lack of quantitative historical information on plankton. Algal blooms have been recorded at the Abbotsbury end of the Fleet on a number of occasions since 1969 (Whittaker 1980, Saunders-Davies 1993, John 1995, EA 1998a), mostly during particularly warm dry summers. Blooms in 1969 and 1976 were associated with fish deaths, and possibly with farm pollution incidents, but it is not known which species of plankton were involved. The species involved in the later blooms varied, and some blooms were not of toxin-producing species (eg. that recorded by John (1995) mentioned above).

In conclusion, the only evidence from the Fleet of detrimental change in conservation interests in response to nutrient levels is:

• a possible decline in the distribution and population of Lamprothamnium papulosum at the western end of the Fleet. Research on the species in the Fleet and elsewhere suggests a critical threshold of soluble reactive phosphate between 10 µg/l-P and 30 µg/l-P.

However, caution is required as there is insufficient information from which to draw conclusions with respect to:

• increases in growth of epiphytic algae;
• increases in growth of benthic green algae;
• increases in phytoplankton blooms;
• impacts on fauna.

The known and potential impacts of the first three of these on conservation interests, particularly seagrass Zostera, are described in another section although there is little evidence to identify critical thresholds of nutrient concentrations with respect to deleterious increased growth in epiphytic algae, benthic algae and phytoplankton. These impacts, and the lack of information, indicate that it is not possible to conclude whether there has or has not been an impact on conservation interests.

Indications of intensification of agricultural land use around some parts of the Fleet led to concern that this might result in increased nutrient inputs to the Fleet from surface water run-off and thus more frequent algal blooms and increased green algal growth. The work by EA since 1996 on nutrient concentrations of inputs to the Fleet as well as the Fleet itself (EA 1998a and 1998b) has indicated that the Fleet and its freshwater inputs do have high nutrient concentrations. However, there is no clear evidence of increases in nitrogen or phosphorus concentrations, primarily due to a lack of sufficient data on past water quality to allow identification of trends. Nutrient concentrations found by John (1995) and the Environment Agency (EA 1998a) are broadly similar, with the exception of the very high nitrate concentration (372 mg/l-N) found by John (1995) on one occasion in Mill Stream (Abbey Barn). Because of the small number of measurements, no firm conclusions can be drawn as to whether nutrient concentrations have increased since 1995. It is also not clear whether algal blooms are increasing or decreasing in incidence or intensity.

However, it is unlikely that there will have been a major change in the short time since 1995. To consider trends in relation to changing agricultural practice we really need to estimate nutrient status over the last 50-100 years.

Considering the information that is available, different statements can be made about the eastern and western parts of the Fleet. There would seem to be no nutrient enrichment or potential for eutrophication of the eastern, more marine end of the Fleet. Primary sources of nutrient inputs to the Fleet do not appear to include seawater entering the Fleet. There are very few, or only minor, stream inputs into the eastern Fleet. Run-off from agricultural land is likely to result in nitrogen loading to the east Fleet. However, tidal flushing of this eastern part of the Fleet occurs regularly. It is likely that even if nitrates were not flushed out, they would have little biological effect. This is because the main inputs are in winter, the period during which problematic species, such as phytoplankton, are least able to utilise increased levels of nitrates.

By contrast, however, all the work undertaken during this study indicates that there is a potential problem with nutrient enrichment of the western part of the Fleet, in particular of the Abbotsbury embayment. The major sources of nutrient inputs occur to the western part of the Fleet, with the Abbotsbury embayment receiving diffuse inputs from agriculture, and direct inputs from three streams, two of which include treated sewage effluent, as well as being the location of the swannery. In addition, flushing of this part of the Fleet by seawater and, particularly during the summer months, by freshwater is very poor. There is also a peak in inputs (of phosphorus) during the summer. As a result of these factors, there is a high potential for plant and algal species of concern, such as phytoplankton, to increase in numbers due to these nutrient inputs. Furthermore, winter inputs of phosphorous may be indirectly utilised as a result of poor flushing, incorporation into the sediment and subsequent uptake by plant and algal growth during the spring.

It is likely that increases in nutrient inputs to the western Fleet have caused or will cause increases in epiphytic algae on seagrasses, green algal mats and phytoplankton populations. Any such increases are likely to be detrimental to the health of:

• eelgrass and tasselweed beds by increased epiphyte and benthic algal growth, and increased water turbidity restricting light availability to the plants;
• foxtail stonewort (Lamprothamnium papulosum) due to increased competition from green algae and direct effects of increases in phosphate concentrations;
• lagoonal invertebrate and fish populations by increasing the likelihood of harmful anoxic conditions near the sediment surface and in the water column overnight due to increased green algal growth, and by algal blooms in warm conditions either being directly toxic to invertebrates or fish, or by reducing oxygen available to them during post-bloom decay of algal cells.

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