Nutrient levels associated with ecological thresholds of impairment: An approach to estimate numeric nutrient criteria for reservoirs of Puerto Rico

Abstract

Six reservoirs of Puerto Rico were monitored over a 32-month period to establish relationships between their nutrient concentration status and different thresholds of ecological impairment. The selected reservoirs embody the productivity spectrum of reservoirs on the island. Median concentrations of total phosphorus (TP) for epilimnion waters (1 m) were as follows: Cerrillos, 10 µg/L; Cidra, 33 µg/L; Guajataca, 10 µg/L; La Plata, 49 µg/L; Patillas, 6 µg/L; and Toa Vaca, 22 µg/L. Likewise, median concentrations of total nitrogen (TN) were:Cerrillos, 0.22 mg/L; Cidra, 0.74 mg/L; Guajataca, 0.28 mg/L; La Plata, 0.55 mg/L; Patillas, 0.23 mg/L; and Toa Vaca, 0.34 mg/L. Strong positive correlations were observed between nutrients and chlorophyll a (Chl-a). Correlation coefficients were 0.74 for the relationship between TP and Chl-a, and 0.66 for TN vs. Chl-a. Increases in algal biomass (as measured by Chl-a) diminished the depth of light penetration into the water column (i.e., Seechi depth) (r = -0.70). A biological threshold indicative of designated use impairment was established at a Chl-a concentration of 24 µg/L based on evidence demonstrating that the extent of reduced light penetration at that Chl-a level (due to excessive phytoplankton biomass productivity) hindered the reservoirs' capacity to comply with the aquatic life criteria. Total phosphorus and TN concentrations associated with the Chl-a impairment threshold (i.e., 24 µg/L) were established based on a change-point analysis of the data. The resulting values, 0.035 mg/L for TP and 0.43 mg/L for TN, can be considered nutrient thresholds associated with impairment. To protect against impairment, a margin of safety was added based on concentrations defining the lower 5% confidence interval of the bootstrap distribution of values associated with the Chl-a impairment threshold (i.e., p<0.05-one tail probability). The resulting criteria, 0.026 mg/L for TP and 0.41 mg/L for TN, are proposed as basis for establishing the nutrient standards for reservoirs of Puerto Rico. Significant differences in the maximum depth of compliance with the USEPA aquatic life criteria (i.e., DO = 5.0 mg/L) were observed between data points below and above the proposed numeric criteria. These differences, which can be critical to the sustainablllty of fish populations in reservoirs on the Island, attest to the importance of maintaining the nutritional status of waters in our reservoirs at levels below the specified thresholds. The zooplankton community was characterized by a mix of species already known under similar environmental conditions in the Neotropical region. Sixty-five zooplanktonic taxa were identified. Rotlfera was the richest group with 37 taxa. Cladocerans were represented by 20 taxa; Copepoda by eight taxa. Absolute abundances of rotifers showed an increase with the increase of several eutrophication indexes. A segmented curved analysis was used to describe the relationship between Brachionus rotifers (#ind/L) and phosphorus (lnTP). The relationship yielded a change-point value of 26.58 µg/L for total phosphorus, which coincides with the proposed numeric criteria value. In addition, a change-point of 1.16 meters was obtained for the relationship between Brachionus rotifers (#ind/L) and Seechi depth (m), which can be considered as another impairment index for reservoirs on the island.