Te Waihora (Lake Ellesmere) is a shallow coastal lagoon located on the east coast of the South Island of New Zealand. An extensive macrophyte bed (primarily Stuckenia pectinata and Ruppia spp.) once existed around the lake margins. It disappeared after a storm event in 1968 and never returns. Now the lake is highly turbid and water column attenuates light rapidly through depth. The lake is periodically open to the sea, and salinity in the lake fluctuates as a result of the lake opening activities, and varies with distance to the lake opening. The aim of this study is to define a habitat template, a set of environmental factors (light attenuation, water depth, salinity) that allows S. pectinata to grow and persist indefinitely. A series of experiments were conducted to investigate: 1) the morphological and leaf photosynthetic physiological acclimation to low light conditions; 2) the leaf demography and leaf photosynthesis in direct and stepwise exposures to a gradient of salinity; 3) the interaction of light acclimation and salinity on leaf photosynthesis.

At low irradiance, primary shoots of S. pectinata grew taller, produced longer and lighter leaves and senesced leaves more quickly than at high irradiance. Salinity stress induced early leaf senescence and leaf production rate decreased with increasing salinity level. Plants did not tolerate salinity exceeding 20 ppt. Leaves at low irradiance havd higher chlorophylls per fresh weight (per photosynthetic leaf area), suggesting enhanced light harvesting (capture of photon energy) efficiency per unit leaf area. Leaves at high irradiance had fewer chlorophyll per fresh weight but higher photosynthetic rate per unit chlorophyll, indicating higher light utilization (utilization of captured photon energy) efficiency. High salinity (=> 12 ppt) reduced photosynthetic rate per unit chlorophyll-a in leaves acclimated to high irradiance (340 μmol/m2/s) but not to lower irradiances (110 and 50 μmol/m2/s), suggesting salinity would reduce light utilization efficiency of surface reaching leaves but not submerged leaves in the turbid water column.

To conclude, turbidity and salinity synergistically limit the growth of S. pectinata in the lake:

1. both low-light stress in the water column and salinity reduce the number of leaves, therefore, less photosynthetic leaf area; 2) the photosynthetic rate per unit chlorophyll-a is either constrained by low-light stress in the water column or by salinity at or near the water surface. The knowledge of light and salinity limitation on growth of S. pectinata seems to well explain the dynamic of a population of S. pectinata in Te Waihora monitored over two consecutive growing seasons. In the end, a habitat template with defined ranges of water depth, light attenuation coefficient, and salinity was constructed for S.pectinata to grow in Te Waihora.