A summary of MSc dissertation by Nina Lyman (summer 2005)
There is little information concerning aquatic invertebrates or their relationship with wildfowl in the lakes at Paxton Pits. There is also little information on the water quality. The aims of this study were to
1) provide baseline data of freshwater aquatic invertebrates present in the SSSI- designated lakes at Paxton Pits Nature Reserve
2) measure factors influencing the diversity and evenness of freshwater aquatic invertebrates
3) measure water quality (with the use of biotic indices) and its influence on aquatic invertebrates
4) understand how aquatic invertebrates influence the abundance of wildfowl.

Invertebrates were sampled from Cloudy, Hayling, Rudd and Heronry South lakes within Paxton Pits Nature Reserve. Heronry North, Washout and Island lakes were also sampled as these are within the Site of Special Scientific Interest and therefore are of conservation priority.

Invertebrates were sampled from 6th-29th June 2005. To represent the entire invertebrate community of each lake, samples were taken from the middle as well as the edges. In total, six sites per lake, three around the edge (shallow water) and three in the middle (deep water) were selected. Benthic macro-invertebrates were collected using a standard pond net after the sediment and vegetation was agitated using the edge of the net. The net was swept in one motion from the surface of the water, throughout the water column, and then scraped along the top layer of sediment. This action repeated for 30 seconds was sufficient to collect an adequately sized sample. Eight sweeps were taken in total in the same place of each chosen site. Where water was deep and the pond net could not reach the bottom of the lake, it was swept throughout the water column to a depth of about two metres for 30 seconds.

The vegetation survey was conducted midway through sampling on 15th June. Percentage cover was estimated for each lake under the categories submerged, emergent, floating and overhanging to quantify how uniform in species each lake was.

A total of 20,339 individuals in 49 taxa were collected during the study period. Cloudy Lake held the greatest diversity of aquatic invertebrates (from 37 families), followed by Hayling, Rudd, Heronry South, Heronry North, Washout and Island Pit (27 families). The majority of lakes studied at Paxton Pits show high levels of invertebrate diversity and evenness, with the two Heronry Lakes holding the greatest number of invertebrates in shallow water. Total chironomid (water mite) abundances are exceptionally high in Heronry South, more than twice the number in Heronry North or any of the other lakes sampled.

Results show the abundance of invertebrates to be greatest at the edges of the lake in shallower water. This is not surprising as the edge of lakes, known as the littoral zone, are typically nutrient rich and dominated by aquatic vegetation. This zone is where coarse particulate organic matter from shoreline trees accumulates, adding to the large quantities produced by aquatic vegetation. The littoral zone therefore supports the greatest abundance of invertebrates as organic matter provides food and shelter for them.

Heronry North and South have the highest abundance of invertebrates in shallow water. It is possible that the high guano input from the large population of Cormorants nesting in the overhanging willows is increasing the organic content of the water and reducing levels of dissolved oxygen. Low oxygen is, however, unlikely to be a problem because Heronry South also has the highest total abundance and a relatively diverse community of invertebrates. Sufficient oxygen must therefore be present to support them.

Analysis shows Hayling and Rudd Pits to have the most pollution-sensitive invertebrates of the lakes sampled. Cloudy Pit and Heronry North and South were shown to be relatively similar in quality. Washout and Island Pits had slightly fewer pollution-sensitive invertebrates. The overall quality ratings of the lakes sampled range from 7 (Hayling and Rudd) to 5.5 (Island). All lakes sampled in the study are therefore of excellent water quality.

Of the six factors included in the analysis, only gravel extraction technique was shown to influence the diversity of invertebrates. Lakes extracted using the old technique of wet dredging (Hayling, Rudd, Cloudy, Heronry South) are 1.7 times more diverse in invertebrates than those extracted using wet suction (Heronry North), and that lakes extracted using the newer technique of dry digging (Washout and Island Pits) were 1.1 times more diverse than wet suction. Wet dredging was 1.5 times more diverse in invertebrates than dry digging. Extraction type influences depth, sediment type, age and jaggedness, and thus the vegetation that colonises the lake.

Invertebrate diversity and evenness do not appear dependent on water quality for high diversity or evenness, but it is important to remember that all the lakes sampled had good water quality.

Correlations for both breeding and over-wintering wildfowl show that invertebrate abundance and not diversity or evenness appear to influence the number of wildfowl in each lake. The numbers of wildfowl broods are significantly correlated with the number of molluscs: Hayling Pit has the highest total number; Heronry South and North also show relatively high mollusc abundance. There was a significant correlation between total summer invertebrate abundance and Tufted Duck counts the previous winter. There was no significant correlation between the number of wildfowl broods and total chironomid abundance for each lake.

To improve invertebrate abundance and diversity, and increase the availability of invertebrates to wildfowl, it may be useful to increase the amount of shallow water. Gently sloping shores are attractive to invertebrates as well as wildfowl.

The invertebrate survey also identified a nationally scarce species of diving beetle Ilybius fenestratus (family Dytiscidae) in Rudd Pit, Cloudy Pit and Heronry North. It will be important to conduct autecological research in order to determine the importance of particular habitat features. It is vital that the species is included in the Management Plan for Paxton Pits Nature Reserve. The distribution and abundance of Ilybius fenestratus should be monitored annually to determine the success of any management actions or the effects of natural changes. Any newly discovered sites could then be managed alongside existing sites to maintain a viable population and possibly expand the range.