Ecology of eelgrass, Zostera novazelandica Setchell, in Otago Harbour, Dunedin, New Zealand

Abstract

The seagrass Zostera novazelandica Setchell forms extensive intertidal beds at Harwood in Otago Harbour, south-eastern New Zealand. However, despite being an important feature of the Harbour ecosystem, knowledge of spatial and temporal patterns, and of the ecology of this seagrass habitat is very limited. Major aims of this study were to map the distribution and areal extent of Z. novazelandica, and to understand how temporal changes in seagrass growth and production are controlled by environmental factors.

Thematic maps of the seagrass habitat area were developed using digitised aerial photographs and image processing techniques. Seagrass covered about 80 ha of the intertidal area, of which most was assigned to a 'sparse Zostera' category. Integration of these maps with a geographic information system enabled the detection and quantification of areal changes that occurred between April1997 and April1998.

Field studies carried out in permanent plots at Harwood during the period October 1996 to December 1998 documented defined seasonal changes in the rates of leaf growth and primary production, above-ground biomass, leaf length and leaf area index of Zostera novazelandica. These growth parameters were typically higher in summer and lower in winter. Biomass of above-ground parts showed a 2-fold increase from winter/spring values (mean of 40 g DW m-2) to summer values (mean of 97 g DW m-2). Mean leaf proportional growth rate, which was estimated using a leaf-marking method, ranged from 0.005 g DW g-1 day-1 in winter to 0.028 g DW g-1 day-1 in summer. The mean primary production rate varied from 0.2 g DW m-2 d-1 in winter to 2.0 g DW m-2 d-1 in summer. Results from a multiple regression analysis indicated that the variation in above-ground biomass was controlled mainly by air temperature. The seasonality in leaf proportional growth rate was, however, significantly influenced by air temperature and photon flux density (PFD). These findings were supported by the results of controlled laboratory culture experiments in which leaf growth rate was not influenced by the interactive effect of seawater nutrient enrichment with PFD and temperature. Furthermore, leaf growth rate did not respond to artificially elevated pore-water ammonium during a summer in situ enrichment study. Z. novazelandica, however, responded to the ammonium enrichment by increasing the canopy height, below-ground biomass, and chlorophyll a and b concentrations. These findings may indicate that growth of Z. novazelandica is nutrient limited during the summer.