PRODUCTION OF ISOETES-LACUSTRIS IN A PYRENEAN LAKE - SEASONALITY AND ECOLOGICAL FACTORS INVOLVED IN THE GROWING PERIOD

A leaf marking method was applied for the measurement in situ of the p rimary production of Isoetes lacustris L., supplemented by the descrip tion of the leaf growing pattern. Leaf production in this species in L ake Baciver was highly seasonal with maximum values in July (0.36 g cm -2 day-1 at 0.8 m and 0.93 g C m-2 day-1 at 2.3 m depth) and no winter production. Total annual production ranged from 75 g dry weight (DW) m-2 year-1 in the shallow water population (0.8 m) to 2 26 g DW m-2 ye ar-1 in the deep water population (2.3 m). The increase in production. with depth appeared to depend directly on plant biomass. The low water temperatures and extremely reduced light levels prevented leaf produc tion in Isoetes lacustris from mid November to early May. In June, pro duction per irradiance unit was very low, which could be related to gr owth limitation by temperature, as shown by light and temperature resp onse experiments. In situ fertilization of the sediment and a decrease in nitrogen and phosphorus concentration in tissues during the growin g period showed that growth of this species in the lake is nutrient li mited. The mean annual C:N:P (molar) ratio 525:26:1 suggests a strong phosphorus limitation for Isoetes lacustris in Lake Baciver.