A study of sediment formation, transport and deposition in Lough Neagh, Northern Ireland, with special reference to diatoms

Abstract

1. This project was initiated to ascertain the accuracy with which seasonal changes in seston concentration and composition are subsequently recorded in the accumulating sediments of Lough Neagh, Northern Ireland.
2. 2. A set of sediment traps was deigned, constructed and deployed to intercept sediment material (SM) at two locations in the lough, the Battery and Antrim Bay sites. These traps were cylindrical, 8cm high and 31.5 com in diameter (height/diameter ratio (the aspect ratio) = o.25) and were positioned one metre above the natural sediment surface on metal frame. Triplicate set of traps were routinely sampled at intervals which ranged from two weeks to one year. The integrated sampling programme extended over two years (1 May 1976 to My 1978).
3. The gross sedimentation rate, as recorded at two-weekly intervals, showed a seasonal pattern with high rates occurring during winter and spring months (September to May0. In both 1977 and 1978 the highest rates occurred in February, march and April. Winter SM (September to March) was characterized by a high content of classical material and relatively low numbers of diatoms, but periphytic species were proportionally more abundant than at other times in the year. Both the influx rate and the concentration of diatom valves in SM increased greatly in the spring SM (April and may), immediately following the decline of the crop in 1977 and 1978. Summer SM (June to August) had high organic content (up to 41%), but very few diatoms. 1976 was unusual in that a ‘normal’ spring diatom crop failed to develop and most diatoms in SM collected during this year were derived predominantly from sediment resuspension.
Blue-green algae were not collected in large numbers in the traps, even following drop decline in the late summer. It is thought that the blue-greens are mineralized almost entirely within the water column in Lough Neagh.
4. Simple and multiple regression analyses of the SM influx data from the Battery site with water column and metrological variables were performed. The rate of gross sedimentation shown to be very significantly correlated (p = < 0.001) with both of the following predictor variables, standing diatom crop and wind speed, in the preceding SM sampling period. Both the influx of planktonic diatoms to the traps and the proportion of living Stephanodiscus Astrea in SM were very significantly correlated (p = < 0.001) with the standing crop in the preceding SM sampling period. With the exception of the spring diatoms, periphytic diatoms and clastic material were very highly significantly correlated with each other (p = < 0.001) and also with the wind speed and rainfall data (p = < 0.02). Again, with the exception of the spring diatom bloom period, no correlation was found between the influx rates f these variables and diatom crops and it is thought that SM at these times consisted mainly of redeposited and inwashed sediment. This sediment appeared to be largely composed of material resuspended from the deeper, central areas of the lough.
5. Measurement of the biogenic silica content of SM and sediment by Na2co3 digestion was carried out to investigate silica fluxes in the lough. Following ‘correction’ for redeposited sediment, good agreement was found between the quantities of biogenic silica available for sedimentation at the time of the diatom crop maxima in 1977 and 1978 and the quantities deposited on the sediment surface immediately after bloom. The annual influx of biogenic silica, a measured in sediment cores collected in September 1977, was similar to the quantity estimated to have been deposited after the diatom bloom in April of that year. Assuming that diatoms deposited in the non-sediment accumulating area of the lough (c. 40%) supplemented those deposited in the deeper, sediment accumulating zones, then about 30% of the initially deposited diatoms was accumulated to be released back into water column as dissolved silica.
6. SM collected from the batter and Antrim Bay sites was shown to be similar in composition and influx rate. However, a positive correlation between magnetic susceptibility of Antrim Bay SM and rainfall indicates that variations in catchment erosion were reflected more directly in the SM at this site, as would be expected from its proximity to river inflows.
7. Sediment trap design and length of exposure period were found to be related to the quality of SM collected. Traps with a high aspect ratio (11.76) were found to lose little sediment by resuspension during storm period and collected about x10 more SM than low aspect ratio traps (0.25) over a yearly period. Extending the traps exposure period decreased the rate at which a trap collects SM. Traps of low aspect ratio (0.25) exposed for yearly periods collected SM at a rate (1.18 cm/yr) nearest to that measured in sediment cores (0.83 cm/yr).
8. Results from the sediment trapping and coring programmes showed that the abundance and species diversity of planktonic diatoms in SM accurately reflected, on a seasonal and annual basis, changes in the diatom phytoplankton in the water column of Lough Neagh during 1977 and 1978.

Date of Award	Feb 1980
Original language	English