engleski

Using phytoplankton bioassay to assess nutrient enrichment in the waters surrounding a fish farm

Increased concentration of nutrients resulting from fish farm operations may cause eutrophication which could lead to negative impacts on the environment. Due to the dynamic nature of dissolved or suspended compounds in the water column, one of the major challenges is the measurement of the flux of nutrients emitted from fish farms. Traditionally, nutrient concentrations are measured by discrete sampling in the water column, but diurnal measurements (Angel, unpublished) have shown that concentrations may vary by as much as an order of magnitude or more, depending upon when we sample. Clearly, a better approach is needed to obtain conclusive information regarding nutrient flux from fish cages. Phytoplankton bioassays offer a low-cost alternative monitoring strategy to determine the sphere of influence of fish farms on surrounding water quality (Dalsgaard and Krause-Jensen, 2006). Dialysis bags filled with seawater are deployed for 5 days near the sea surface along a transect stretching away from the fish farm. Due to constant flux of nutrients across the dialysis membrane, growth of phytoplankton within the bags should reflect the integrated ambient nutrient concentration field at the respective deployment stations. Dalsgaard and Krause-Jensen (2006) used final concentration of Chlorophyll-a to determine fish farm impact area. However, their approach only determinates boundaries of impact and not the nutrient concentration field. We report on an ongoing effort to calculate phytoplankton growth rates and to estimate the nutrient concentration field in the region surrounding fish farms using the phytoplankton bioassay. In June 2006 we deployed a phytoplankton bioassay on Dalmar Ltd. sea bream/sea bass fish farm near Pakoštane, Croatia. We filled 32 dialysis bags with surface seawater from a reference station 1200 m upstream from the fish farm. We used 25μm sieve to remove grazers and large phytoplankton from 16 bags, obtaining two sets of samples – unfiltered and filtered. Four bags of each set were deployed for five days at four stations (reference station, and 0, 25 and 50 m downstream of the fish cages). The samples were analyzed for Chlorophyll-a concentrations using HPLC. The results show that during our 5 day deployment Chlorophyll-a concentration increased 12-30 fold in unfiltered samples and 10-54 fold in filtered samples. The unfiltered samples do not show any significant difference among stations while filtered samples show significant difference between the reference and other stations. We modeled phytoplankton growth to estimate nutrient concentrations at the experimental locations. Since dialysis bags allow flow of nutrients while retaining the phytoplankton, we approximate the bag with a well-mixed reactor. Nutrient concentration in the reactor is then a function of the input nutrient concentration and uptake by the phytoplankton. The input is equal to the nutrient concentration, and uptake by the phytoplankton is determined by our model. The model uses Monod’s function for nutrient uptake and Droop’s function for growth. We arrive at the nutrient concentration by requiring that the final predicted Chlorophyll-a concentrations match those observed in filtered samples in our experiment. We calibrate our results using theoretical estimates of nutrient distribution obtained with KK3D model previously developed by Jusup et al. (2007).

bioassay; nutrient enrichment; phytoplankton growth dynamic; fish farm

nije evidentirano