Hydrodynamics of an isothermal lake, L. Neagh, with particular reference to the effect of waves

Abstract

The major aim of the study was to collect information on physical processes relevant to ecological studies in Lough Neagh, a large (387 km2) shallow lake (mean depth 8-9 m). The lake is isothermal for most of the year, with a seasonal temperature range between 3-8 and 20-8°C. Winds were predominantly S-SW (mean speed 4-78 m s·1, max. 18 m s· 1 ) but it is characteristic of the lake that winds are very variable in duration, direction and speed. Surface wave properties and wind drift currents were measured at a range of inshore and offshore sites in different wind conditions. Inshore current speeds were up to 16-65 cm s· 1 at 1-0 m depth (in wind speed 7-7 m s· 1) and, unless affected by wave action, tended to be more stable in direction than offshore currents, which were highly variable with depth and time, ranging between 1-2 and 6-93 cm s· 1. Deep water bottom currents (12m) were also stable for long periods in summer, reaching 7 cm s· 1. Significant wavelengths (periods) of 7 - 42 m (2 - 5 s) were measured in wind speeds 6-2- 15-4 m s·1 and effective fetches were between 0-7 and 18-4 km. Measured wavelengths agreed well with predicted maximums but exceeded time-adjusted predictions. Estimates of the depth of the wave-mixed layer along selected transects showed that in high winds most of the water column can be affected by wave action. The extent of the shore zone was estimated in a variety of wind conditions. In one storm (wind speed up to 15-0 m s-1>, 91-9% of the lake bottom was affected by wave action. The implications of this for the biology of the lake are discussed. For example, in each of three years studied, the extent and location of bottom areas disturbed by wave action were very different (43-2 %, 91-9 %, 74-1 %). This resulted in considerable differences in the number of diatom cells resuspended at the beginning of the growth period in each year. So, differences in timing of physical processes can have a significant effect on phytoplankton population dynamics.

Date of Award	Jan 1993
Original language	English