Physiological and ecological aspects of photosynthesis by the plankton in Lough Neagh with particular reference to light

Abstract

Physiological and ecological aspects of the phytoplankton of Lough Neagh are discussed. In particular the photosynthetic rate is considered in respect of its response to light, as the energy source, with its variation in time and space. Spectral distribution with depth was measured using coloured glass filters and a narrow band width spectroradiometer. Light is mainly attenuated by phytoplankton pigments and coloured dissolved organic compounds. Variation in the depth of the euphotic zone was relatively limited ( = 1.5 meters) throughout the year. The changes are largely due to the phytoplankton which are self-shading at high densities but at lower crops the dissolved organic compounds increasingly became the dominant factor in light extinction. The resulting ratio of illuminated to dark regions of the water column are discussed in terms of the effect this has on autotrophic growth. Photosynthesis was measured by the evolution of oxygen and the uptake of 14c in light and dark bottles. The algae circulate freely in the water column for most of the year, so experiments were initiated to study the effect of this on integrated values of gross photosynthesis per square metre. The algal crops in the open lake varied between 25-90 mg chlorophyll am-3 with spring crops of Melosira italica being replaced by the blue green alga Oscillatoria redekei which was present in considerable quantity throughout the period of study. Algal growth reduced nitrate and phosphate to low levels in summer. Higher crops (up to 300 mg chlorophyll a m-3) were found in a shallow almost enclosed area, Kinnego Bay, where nutrients a are generally higher due to the inflow of storm water and poorly treated sewage from the local sewage plant. The controlling factors of photosynthesis in this area, with its higher crops but shallower mixed depth, are compared with the open lake. Gross photosynthesis at light -3 -1 saturation of 200mg Cm -3hr-1 for Lough Neagh and 700mg Cm-3hr-1 for Kinnego Bay, giving daily integrals of up to 5g cm-2day-1, show Lough Neagh to be among the most productive of temperate lakes.

Date of Award	Nov 1973
Original language	English