Effects of Inorganic Phosphorus and Nitrogen on the Growth of an Estuarine Cladophora in Culture

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Abstract
Response of Cladophora to different P and N regimes under otherwise non-limiting conditions in cultures of artificial seawater suggests that growth rates would be limited in the field by P and N concentrations in the water column above the algae, but not in the algal bed. Treatment with P over the range 0-0.25 mg l-1 (with 5.0 mg l-1 N) and N over the range 0-5.0 mg l-1 (with 0.5 mg l-1 P) produced yields proportional to increasing concentrations. Growth rates were obviously saturated above 0.4 mg l-1 N and 0.2 mg l-1 P, and the response was most dramatic in substrate concentration ranges of 0-0.1 mg l-1 N and 0 to 0.03 mg l-1 P. Uptake rates of P increased in proportion to substrate P concentrations, with the highest rates occurring immediately after conditioning in P-free medium. Rates over a combined light and dark period were typically lower than those measured in the light, which gave a vmax of 0.11 mg g-1 dry weight hr-1. Ammonia and nitrate were taken up simultaneously at all concentrations, in the presence of 0.5 mg l-1 P. There was a clear preference for ammonia, with suppression of nitrate uptake at the highest N substrate concentration. Uptake rates of both N species was highest in the light, with a vmax 1.82 for NH4 and 0.59 mg/g dry weight.cntdot.h for NO3. Tissue concentrations of N and P increased in proportion to the substrate concentration; the relationship was linear except for high N substrate concentrations, where there was loss of organic N from the tissue. Relating the growth-saturating N or P concentrations in the water with their corresponding tissue N or P concentrations suggests that the critical tissue concentrations for growth are about 21 mg/g N and 3.3 mg/g P. Tissue levels well above this critical concentration were obtained for those algae treated with very high N or P, suggesting that the alga has the capacity to build up its N and P reserves to very high levels in the field under non-limiting environmental conditions. The minimum viable tissue content was estimated for N at 12 mg/g dry weight, and for P at 0.5 mg/g dry weight. A comparison with tissue levels measured on field-grown material suggests that there is luxury storage of N and P. Removal of either nutrient from solution, after growth in complete medium, resulted in a marked reduction in growth rate, with a concomitant decrease in levels of tissue N and P after 4 wk, particularly for those algae without added N. Those which had been incubated in higher concentrations grew more vigorously on return to minimal medium. There was a strong correlation between tissue N levels and chlorophyll a, though this was less obvious with P. Removal of N from the medium resulted in distinct chlorosis of algal filaments after 5 wk, suggesting that N availability may well be important in the interpretation of differences in thallus color.