Aquatic plant decline in Lake Ellesmere : a case for macrophyte management in a shallow New Zealand lake : a case for macrophyte management in a shallow New Zealand lake : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the University of Canterbury [Lincoln College]

Abstract

The biology and ecology of aquatic Ellesmere, a shallow coastal lake submerged plants were studied in Lake in the South Island of New Zealand. The aim of the study was to understand previous declines of macrophytes in the lake as well as their non-regeneration after a major disturbance event. Such an understanding was assumed to play a central role in the overall management of the lake.

Declines of aquatic plants were first reviewed world wide to identify potential sources of perturbation or stress for macrophyte communities. In Lake Ellesmere, wave action, grazing, sediment accretion, water level fluctuations, inorganic and organic turbidity, and salinity were identified as having a potential negative effect on plant growth.

The effects of salinity and light on the growth of Ruppia megacarpa and Ruppia polycarpa seedlings were investigated in glasshouse experiments. Both factors were shown to affect growth. Poor light reduced rhizome elongation which may prevent the anchoring of seedlings that are continuously exposed to a turbid and turbulent environment. Growth was also found to be reduced under salinities above 10 PPT. Such salinities occur in the field after prolonged lake openings.

A study of the underwater lake environment showed that inorganic suspensoids were mainly responsible for the high turbidity of the lake. About 40% of the reduction in light penetration in the water column was due to shading by phytoplankton. Levels of inorganic suspensoids were highly correlated to wind velocity. Relationships between clarity determinants and optical properties were also established. Their potential use for future management is evaluated.

Standing crops of the main species were obtained from two areas in the lake in the summers of 1985-1986 and 1986-1987 during a sudden period of macrophyte regeneration. This allowed the potential for aquatic plant reestablishment to be assessed. Recent developments in ecological theory and plant population biology were used in this assessment and proved to be very valuable.

Measures that could favour re-establishment are presented and their integration in future especially in regard lake management is discussed. A new strategy, to lake level fluctuations, was found desirable. Experimental management, where any alternative measure is in the nature of an experiment rather than being a programme with predetermined results, is finally suggested as a very appropriate approach to macrophyte management that may reduce uncertainty in the future management of shallow lakes.