Undergraduate Winner: Charlotte Thompson, Southampton Oceanography Centre, University of Southampton

The Effect of the Presence of Cockle Shells on the Erosion of a Cohesive Sediment Bed

An annular laboratory film has been used to investigate the effect of the addition of cockle shells to the erosion of a cohesive sediment bed. A standard clay bed was rolled and shells of differing sizes added to analyse any changes of erosion rate or suspended sediment concentration. Corrasion has been a suggested mechanism for this erosion, however, it was found that cockle shells travel in bedload transport along the bed, and never enter into the suspension as was expected. This was found to be due to large amounts of energy required to lift a shell into suspension never being reached. The behaviour of the shells while travelling around the flume, was found to be directly related to their behaviour while settling in still water. The addition of the shells was found to be the factor that induced erosion, as the flow velocities alone were not enough, and the larger the physical dimension of the shell, the higher the erosion rate was found to be. The erosion rate was found to be the result of abrasion, rather than corrasion. There was a linear increase in erosion rate with increasing shell size, and an exponential increase in the suspended sediment concentration.

The presence of large numbers of cockle shells in areas such as Southampton Water and Lymington have suggested that the processes investigated here may be responsible for the erosion regimes in these areas.

Postgraduate Winner: Karen Pye, Unversity of Portsmouth

The effects of eutrophication on the marine benthic flora of Langstone Harbour, South Coast of England

A thesis presented in candidature for the degree of Doctor of Philosophy of the University of Portsmouth

Langstone Harbour, an enclosed, fully marine inlet on the South coast of England, is currently recognised by the Paris Commission (1992) as being possibly eutrophic and supports excessive annual growths of green macroalgae on the extensive tidal mudflats. Whilst these were the subject of numerous investigations during the 1970's and early 1980's, very few more recent studies have been undertaken. The purpose of the present study is, therefore, to reassess and evaluate the role of anthropogenic sources of nutrients, notably from a sewage discharge pipe in the North of the Harbour, on the occurrence, density, distribution and effects of the apparently resultant green macroalgal mats. Field studies were undertaken regularly over 1994-96 at a number of widely distributed sites in the Harbour. A sampling programme determined macroalgal distribution, biomass, productivity, chlorophyll and nutrient content. Macroalgal cover was also assessed using Infra Red False Colour (IRFC) aerial photography, and vegetation maps produced. Correlated hydrological and meteorological data were also obtained, notably for the seawater content of nutrients, chlorophyll and suspended particulate matter (SPM), and the environmental parameters of temperature, salinity, dissolved O2, light penetration and pH. It was hoped that the data obtained would provide a better understanding of the factors determining the growth of the macroalgal mats and contribute useful information towards the implementation of future management strategies.

With respect to hydrological studies summer maximums and winter minimums were experienced for seawater pH, salinity, dissolved oxygen, Secchi depth, temperature, chlorophyll and SPM. Summer minimums and winter maximums were found for rainfall and water column nutrients. When compared to previous data for Langstone Harbour, levels from this study of pH, salinity, dissolved oxygen, chlorophyll, Secchi depth and temperature were considered stable and satisfactory, showing little change. Nutrient concentrations in Langstone Harbour displayed temporal, spatial and seasonal variations. Levels were similar to those given in many literature reports, and whilst they remained considerably lower than those recorded for some regions, maximum levels did reach, and on occasion exceed, ranges reported for locations considered to have a nutrient problem. Therefore, Langstone Harbour was considered to be moderately affected by nutrients. A temporal increase in phosphate was directly related to phosphate concentrations in Budds Farm effluent and spatial differences (North-South gradient, North being higher) related to dilution and dispersion processes of the effluent. Seasonally, nutrients were higher in winter. Average winter concentrations (of all sites monitored, Dec-Jan) of nitrate, nitrite, ammonia and phosphate (respectively) in Langstone Harbour during 1994-5 were 365.5, 9.14, 62.13, 24.79mgl-1, in 1995-6 were 258.67, 9.52, 79.42, 37.78mgl-1, and in December 1996 were 254.27, 12.36, 97.00, 39.73mgl-1. Maximum nitrate, nitrite, ammonia and orthophosphate concentrations of 1.61, 0.88, 0.938, 0.872mgl-1 respectively were recorded in the vicinity of the sewage outfall in October 1996.

Biological studies showed marked local and seasonal variations in the occurrence, chemical composition, distribution and abundance of the macroalgae, which related to substrate, intertidal position, seawater nutrient status and environmental conditions within the harbour. Enteromorpha and Ulva spp. were the predominant algae inhabiting the mudflats, altering seasonally, spatially and temporally. Macroalgal biomass, productivity, chlorophyll and nutrient content were greater in spring/summer than autumn/winter and generally higher at the northern sites. The mean biomass (gm-2 wwt) ranged from 0-3963 with a maximum of 5818 recorded. Relative growth rates (%d-1) during July/August were -0.1 to 3.7 for Ulva, and -3 to 3.1 for Enteromorpha; low compared to other regions. Total chlorophyll concentrations ranged from 0.556-4.140 mg g-1 in the South and 1.576-4.014 mg g-1 in the North. Nutrient levels (%dwt) in Enteromorpha and Ulva (respectively) ranged from 2.00-7.76 and 1.19-5.77 for N, from 0.19-0.51 and 1.1-1.46 for P, and from 22.4-45.85 and 26.17-33.41 for C. These concentrations, similar to those in the literature, have increased since the 1980's, and only current populations away from the nutrient input in the North were considered to be nutrient limited during the early summer months. Black anoxic muds were often associated with the macroalgal mats, and the presence of blue-green algae and macroalgal rope formations on the mud surface were observed in summer. Details of fauna associated with the macroalgal mats were recorded, with particular attention paid to Hydrobia ulvae whose maximum biomass was 1859.4gm-2.

Infra Red False Colour (IRFC) aerial photographs provided an up-to-date account of local macroalgal occurrence and distribution. Generally less surface area was covered by Enteromorpha in 1996 than 1994. Of the total vegetation cover in the 1990's, approximately 49% was Enteromorpha and 15% was Spartina (alone and mixed with other vegetation). Between 1994 and 1996, the actual percentage change of total areas (m-2) covered by each vegetation group suggested prominent increases over the region for Ulva, Fucus and mud, decreases for Enteromorpha, Enteromorpha & Ulva, Zostera & Enteromorpha, moribund Spartina and degenerate Spartina, and no relative change for Zostera, Enteromorpha & Fucus, Spartina & Fucus, moribund Spartina & Fucus and Spartina & upper saltmarsh species. Since the 1980's, Enteromorpha (particularly mixed with other algae) and Fucus (alone and mixed with other vegetation) has increased, and Spartina was found to have decreased, by almost 50%.

By combining hydrological, biological, physical and mapping data, an understanding of the chemical and biological status of Langstone Harbour over the three-year study period was obtained. It is anticipated that this work will serve as an excellent reference source for future studies, which could focus on establishing whether the algal community is self-perpetuating, and if so, what the maximum sustainable biomass is, and how changes in algal biomass will affect the local ecosystem. It is likely that with no change in nutrient inputs to the harbour, the next decade will see an increase in Enteromorpha biomass.