Phosphorus and nitrogen relationships of Cladophora glomerata in two lake basins of different trophic status

Abstract

SUMMARY Cladophora has increased in abundance and cover in the South Basin of Windermere, English Lake District, in recent years. During the growing seasons of 1992 and 1993, the maximum biomass of Cladophora in the South Basin, at nearly 200 g dry weight m‐2, was nearly six times greater and the percentage cover, at 95%, was nearly ten times greater than in the North Basin. Nutrient analysis showed that Cladophora from the South Basin had significantly higher average contents of N than that from the North Basin. The suggested rate‐limiting tissue N content, 1.3% of dry weight, was never reached in the South Basin but was reached in 20% of samples from the North Basin. In contrast, the average P content was not significantly different, at about 0.24% in both basins. The suggested rate‐limiting P content, 0.16% of dry weight, was reached in 31% of the samples from the South Basin and 37% from the North Basin. On average, the standing stock of P in the South Basin was 0.12 g m‐2, twelvefold that in the North Basin at 0.01 g m‐2 and the standing stock of N in the South Basin was 1.81 g m‐2, elevenfold that in the North Basin at 0.16 g m‐2. Maximum standing stocks of P were 0.38 g m‐2 in the South and 0.05 g m‐2 in the North Basin. Maximum standing stocks of N were 5.02 g m‐2 in the South and 0.60 g m‐2 in the North Basin. The maximal rate of PO4–P uptake ranged between 298 and 2949 μg P g‐1 DW h‐1 and the specific affinity varied between 2.9 and 24.3 μg P g‐1 DW h‐1 (mg PO4–P m‐3)‐1. The higher values of both characteristics are within the range which is believed to indicate severe P limitation. Using the nutrient uptake characteristics, rates of uptake at pelagic and littoral concentrations of PO4–P were calculated to be between 0.1 and 1.7% of the maximal rates apart from one site where rates were up to 5.6% of Vmaxi and 40% of Vmaxi based on pelagic and littoral concentrations of PO4–P, respectively. Despite these generally low rates of uptake, calculations suggest they were just sufficient to account for the measured rate of increase of P standing stock, assuming no biomass loss. Similar calculations suggested that pelagic concentrations of NO3–N could support between 21 and 30% of maximal rates and these were between four and six times (North Basin) and twenty‐five times (South Basin) the observed rates of increase in N standing stock. Littoral concentrations of NO3–N were similar and so would have allowed similar rates. Evidence from nutrient tissue contents, standing stocks and rates of uptake suggests that the growth of Cladophora, in both basins of Windermere, is controlled primarily by the availability of P.