, Volume 134, Issue 3, pp 207-213

In situ nutrient assays of periphyton growth in a lowland Costa Rican stream

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Nutrient limitation of primary production was experimentally assessed using an in situ bioassay technique in the Quebrada Salto, a third-order tropical stream draining the northern foothills of the Cordillera Central in Costa Rica. Bioassays employed artificial substrata enriched with nutrients that slowly diffuse through an agar-sand matrix (Pringle & Bowers, 1984). Multiple comparisons of regression coefficients, describing chlorophyll-a accrual through time for different nutrient treatments, revealed positive micronutrient effect(s). Micronutrient treatment combinations (Fe, B, Mn, Zn, Co, Mo, EDTA), supplemented with and without nitrate and phosphate, exhibited significantly greater chlorophyll-a accrual over all other treatments (P < 0.05), supporting over three times that of the control after 14-d of substratum colonization. Neither of the major nutrients (N or P) produced a significant stimulation, although the N treatment displayed ≃50% more chlorophyll-a than the control after 14-d. Similarly, Si, EDTA, and Si + N + P treatments did not exhibit chlorophyll-a response curves that were significantly different from the control. During the experiment, mean NH4-N and (NO2 + NO3)-N concentrations in the Salto were 2.0 µM (28.6 µg · l−1) and 7.2 µM (100.2 µg · l −1), respectively. High concentrations of PO4-P (\(\bar X\) = 2.0 µM; 60.9 µg · l−1) and TP (\(\bar X\) = 3.0 µM; 94.0 µg · l−1) were also found, and consequently low molar N:P ratios \(\bar X\) = 4.7). Despite the potential for N limitation in the system, both N and P appear to be at growth saturating levels. This may be due to micronutrient limitation and/or light limitation of periphyton growth in densely shaded upstream portions of the stream.