The ecology of a saline lagoon in southern England

Al-Suwailem, Abdulaziz Mohammad (1991) The ecology of a saline lagoon in southern England. University of Southampton, Doctoral Thesis.

Abstract

A wide ranging study has been made of the Gilkicker lagoon, Hampshire, involving the history, morphology, sedimentary regime and ecology of the contained organisms. The physico-chemical parameters, benthic and macrophyte-associated macrofauna were sampled monthly for a period of twenty months. The phytoplankton cycle was followed for a period of one year and the distribution and abundance of other primary producers were investigated. Estimates of primary production were carried out. The biomass of the benthic and macrophyte-associated macrofauna was determined and the secondary production of the benthic macrofauna and the three dominant species associated with the macrophytes was estimated. Aspects of the life cycle of the common macrofaunal species were investigated and discussed in relation to other studies. The lagoon is probably of natural origin but has been modified extensively over the past three hundred years by human activities. The macrofauna has a higher species richness, a greater number of lagoonal species and nationally rare species than any other British lagoon of a similar size. However, in comparison with freshwater, marine and high salinity estuarine environments the lagoon fauna and flora are impoverished, with a few species accounting for most of the individuals. This is related to severe environmental conditions in the lagoon which exhibit relatively wide seasonal and daily variations. The lagoon macrofaunal species are however well established and displayed relatively little temporal variations in species composition and abundance. The lagoon has a high primary productivity (8.7mg c m^-2day^-1, max. gross production) of which the macrophytes (primarily Chaetomorpha linum and Ruppia cirrhosa) phytoplankton level found in the lagoon can be linked with the dense growth of macrophytes, which outcompete the phytoplankton for the available nutrients. Consequently, the phytoplankton bloomed late in the season (late summer-early autumn) after the macrophyte growth had ceased. The lagoon phytoplankton was strongly dominated by flagellates and the photosynthetic ciliate Mesodinium rubrum, whereas diatoms were the dominant group in the adjacent sea. Despite the high primary productivity, the benthic macrofaunal biomass and production were low to moderate (3.0-4.9g afdw m^-2 and 8.7-20.6g adfw m^-2y^-1 respectively), in comparison with other productive systems such as estuaries. The possible reasons for this are discussed. Temperature was found to play a major role in the reproduction of many macrofaunal species. Year to year variations in winter temperature affected the reproductive success and population size of a number of species. The species richness of the lagoon flora and fauna, and the abundance, biomass and production of the macrofauna decrease away from the sea-influenced zone. This decrease is related to the increase in variability and unpredictability of environmental conditions towards the lagoon proper. Species with limited physiological tolerance are thereby excluded from the inner part of the lagoon. In contrast the lagoonal species showed an increase in species richness, abundance, biomass and production away from the sea-influenced zone. The study seems to emphasise the width and complexity of interactions within the lagoon ecosystem.

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