5 Nutrient Indicators

The state thresholds used in the chlorophyll a, total nitrogen, and total phosphorus visualizations come from Florida State Surface Water Quality Standards within the Florida Administrative Code.

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5.1 Summary

Daily average dissolved oxygen values were mostly within or above their average ranges in the first half of the year (Figure 5.1); however, daily averages did fall below the average range at Pellicer Creek in January (Figure 5.1 (d)) which had above average and below daily average range. In previous years, dissolved oxygen values at Pine Island (Figure 5.2 (a)) and Pellicer Creek (Figure 5.3 (a)) frequently fell below the 2 mg/L hypoxia threshold, but so far in 2024, there have not been any dissolved oxygen readings below that threshold (Figure 5.2 (b), (Figure 5.3 (b)).

Chlorophyll a values were often higher than average at all stations this year, especially at Pine Island and San Sebastian (Figure 5.4). Annual geometric mean (AGM) chlorophyll a values were high and above the state threshold levels at all stations, except for Fort Matanzas (Figure 5.4). Elevated chlorophyll a was observed at Pine Island (Figure 5.4 (a)) and at Pellicer Creek in Q2 (Figure 5.4 (d)). Annual geometric mean (AGM) chlorophyll a values were high and above the state threshold levels at all stations, except for Fort Matanzas (Figure 5.5), (Figure 5.5 (c)). The AGM will likely change for all stations as more data are added throughout the year.

Total Kjeldahl Nitrogen (TKN) concentrations were higher than average at all stations in January and March, but concentrations were lower in Quarter 2 (Figure 5.7).

Total phosphorus concentrations were below the state threshold at all stations in the first half of 2024 with the exception of February at Pine Island which was missing data (Figure 5.8). Fort Matanzas experienced higher than average TP values for January – March (Figure 5.8 (c)) and San Sebastian had lower than average TP values for February – June. Relatively high levels of orthophosphate were observed at Pine Island and San Sebastian in January (Figure 5.8 (a), Figure 5.8 (b)) and below average levels were found in April – June for San Sebastian, Fort Matanzas, and Pellicer Creek (Figure 5.8 (b), (Figure 5.8 (c)).

5.2 Data

5.2.1 Dissolved Oxygen

Dissolved oxygen at Pine Island has been uncharacteristically low in the past two years.

Pellicer Creek is located within a waterbody with known impairments for dissolved oxygen, chlorophyll a, and fecal coliform bacteria. As such, this station frequently experiences low dissolved oxygen values, particularly in the summer months (Figure 5.3).