Silica and diatom growth in Lough Neagh: the importance of internal recycling

1. Lough Neagh is a large eutrophic lake with a high dissolved silica (SiO2) concentration arising from the basaltic rock in parts of the catchment and the normal winter maximum concentration is over 8 mg L−1. Based on frequent observations between 1974 and 1997, the annual cycles of SiO2 input, uptake and release are explored.
2. Large spring blooms of the planktonic diatoms Aulacoseira subarctica Haworth and Stephanodiscus astraea occur every year and are terminated by SiO2 depletion. Although there are periods when one diatom species has dominated the other, over the period as a whole, the two diatoms appear to be in a stable coexistence.
3. A model of the effect of river inputs on lake concentration shows that without SiO2 release from the sediment, the maximum spring dissolved SiO2 concentration (and hence diatom crop) would be only about one-third of that actually observed.
4. It is concluded that within-lake processes play a large role and are potentially more variable than catchment processes in determining the available SiO2 in Lough Neagh.
5. The role of benthic animals, especially Chironomus anthracinus, in the sediment SiO2 release process is discussed. Field data suggest the SiO2 release rate is highly sensitive to temperature, but this could be partly caused by an interaction between temperature and animal activity.