AQUATIC WEED-BED STRUCTURE AND PHOTOSYNTHESIS IN 2 NEW-ZEALAND LAKES

The nuisance aquatic weed Lagarosiphon major (Ridley) Moss forms dense beds 2-3 m high in New Zealand lakes. This paper investigates environmental conditions and photosynthesis in these weed-beds in two contrasting lakes, Lake Rotorua and Lake Taupo. There was no clear seasonal pattern in weed-bed biomass which ranged from 294 to 424 g (dry weight) m(-2) in Lake Taupo and from 165 to 265 g (dry weight) m(-2) in Lake Rotorua. The proportion of the total biomass in the top 50 cm of the weed-beds (Canopy Index) was between 13 and 68%, with lowest values in spring. Leaf area ratio (LAR) and chlorophyll a were maximal in the canopy just below the weed-bed surface, with LAR values of up to 450 cm(-2) g(-1) (dry weight). The depths into the weed-bed at which the 1% light level occurred were between 1.0 and 2.7 m in Lake Taupo, and 0.5 and 1.05 m in Lake Rotorua. Minimum chlorophyll a values occurred in winter with summer canopy values greater than 800 mg m(-2). Up to 50% of the chlorophyll a was located in the stems. variability in temperatures was less than expected and little vertical stratification was recorded. Dissolved inorganic carbon (DIC) levels were very different between the two lakes, 0.08-0.42 mM (1-5 g m(-3)) in Lake Rotorua vs 0.42-0.92 mM (5-11 g m(-3)) in Lake Taupo. Speciation of the carbon would be expected to differ between the sites owing to differing pH values which were usually below 7.5 in Lake Rotorua and above 7.5 in Lake Taupo. Light saturated photosynthetic rates reported as per unit dry weight, chlorophyll a or leaf area were much lower in Lake Rotorua than Lake Taupo. Depletion of DIC and CO2 occurred over the day in the canopy of the Lake Rotorua weed-bed. This led to a virtual cessation of photosynthesis in the afternoons on four occasions. It is possible that utilisation of HCO3- in Lake Taupo prevented carbon limitation of photosynthesis at that site.