The ecology and morphology of two Stephandiscus diatom species in Lough Neagh (Northern Ireland)

Abstract

The population dynamics and valve morphology of two closely related Stephanodiscus diatom species were investigated in Lough Neagh, a large (386 km2) but relatively shallow (mean depth 8.9 m), hypertrophic lake. Wind generated waves and currents mean that the lake is isothermal for most of the year, with only temporary periods of stratification. Chlorophyll a concentrations ranged from 15 to 115 µg 1-1. The larger StepJumodiscus species investigated in this study was Stephanodiscus neoastraea HAkansson & Hickel. Population density reached up to 1,600 cells mI-1 in March before silicate limitation, then 50% of cells settled to the bottom in approximately 3 weeks. A smaller, Stephanodiscus species, continued to grow after silicate became undetectable, reaching 2,000 cells m1-1 in May and June before numbers declined due to grazing by large populations of Daphnia (over 50,500 individuals m-2).

Cells were able to survive on the bottom in the shallow depths until resuspended during storms. During periods of poor growth, the rate of size reduction increased, therefore shortening the length of the life cycle. Size reduction in cultured populations of S. neoastraea was estimated from reductions in the mean population cell diameter, over a measured number of doublings. In cultures with a doubling time of approximately 1.5 days, cell diameter was reduced between 0.13 and 0.26 µm per doubling. In slow growing cultures (doubling time of 4.2 days), size reduction was significantly higher, ranging from 0.26 to 0.33 µm per doubling. Maximum reductions of up to 1 µm per doubling occurred in the same cultures after a relatively short period of stress.

The two species showed a temporal separation in the timing of their sexual reproduction. The larger species was always first in late summer or in autumn, followed 1 to 3 months later by the smaller species. The separation was the result of differences in the nitrate threshold that acts as the environmental cue for initiating sexual differentiation of cells. In the larger species, the concentrations needed to exceed 10 µM N03-N and for the smaller species 20µMN03-N.

Variation in four morphological features (interstriae, marginal fultoportulae, valve face fultoportuJae and rimoportulae) on the valve face of cultured and natural populations were measured. In both species, the total numbers found were variable, with interstriae and marginal fultoportuJae number related to valve djameter. In Lough Neagh, the full range of diameters found in the larger species was 18 to 65 µm and for the smaller species 10 to 35 µm. The method of expressing interstriae and marginal fultoportulae as the number per 1 µm of valve circumference was tested and the number of marginal fultoportulae per 1 µm was found to be independent of valve diameter in both species. In natural populations, the number of marginal fultoportulae per 1 µm separated 92% of the valves found into the two species.

However, culture studies showed that the potential range for this feature in S. neoastraea was 0.15-0.45 fultoportulae µm-1. This means that the range found in the natural population (0.18- 0.35 fultoportulae µm-1) is only 57% (less than two-thirds) of the range that can be induced in culture. Comparisons are made with the frequency of selected morphological features in other closely related species in the genus.

Date of Award	Sept 1996
Original language	English