An ecological study of Ulva Lactuca L. and other benthic algae on the Avon-Heathcote Estury, Christchurch
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This study comprises investigations into the ecology of the benthic algae on the Avon-Heathcote Estuary with special attention to the influence of sewage discharge on the principal species. Ulva lactuca, the most important, has been described in greatest detail. The work was carried out in two parts, in the field and in the laboratory. In the field from May 1971 to May 1973, monthly sampling was used to relate the spatial and seasonal variation in algal abundance to relevant environmental factors, such as nutrient concentrations, exposure times, substrate availability, current velocities and grazing by Zediloma subrostrata. Algal abundance was measured from per cent cover and dry weight per area. Aerial photography was employed to map the overall distribution of the algae. In the laboratory, the effects of phosphate and nitrate additions and variation in temperature on the growth of Ulva lactuca were studied. The dominant alga was Ulva lactuca L. which occurred as a small attached winter plant, a larger ribbon-like spring plant, a sheet-like detached summer plant and a smaller bullate detached plant. The latter had previous been described as Ulva laingii Chapman but appears to be the result of low salinity on aging U. Lactuca thalli. The distribution of Ulva and Enteromorpha ramulosa was restricted to the mid-tide zone due to intolerance of long exposure and low salinities. Attached algae were restricted to stable substrates while drift algae accumulated in areas with low current velocities. Eutrophication, resulting from sewage discharge, promoted productivity in the western region of the Estuary and appeared to be the major cause of the increased productivity over the last 40 years Temperature emerged as the main factor determining seasonal growth patterns. Aerial infrared photography was a successful survey tool and allowed the areas of active algae to be detected.