3  2025 Results Summary

Results from 2025 indicate that all RA bay segments met chlorophyll-a thresholds accepted by the FDEP to maintain FDEP Reasonable Assurance for Tampa Bay and to comply with the EPA TMDL (Figure 3.1) and estuarine numeric nutrient criteria for Tampa Bay (EPA Amended Approval Letter Jun. 28, 2013). A water quality dashboard (https://shiny.tbep.org/wq-dash) is available to further synthesize available data, assess additional water quality metrics (phytoplankton counts), and inform Consortium participants and other resource managers on the status of water quality in Tampa Bay. Chlorophyll-a concentrations were notably lower than the previous year for many bay segments likely because of below average rainfall in 2025 (Figure 3.3). Blooms of harmful algal species (e.g., Karenia brevis, Pyrodinium bahamense) were not observed in any of the bay segments. Although understanding and mitigating blooms of Pyrodinium bahamense in OTB continues to be a focus of research efforts, summer concentrations in Old Tampa Bay were lower compared to prior years and the chlorophyll-a criteria was met for the fourth consecutive year of the RA period. Recommendations from the assimilative capacity assessment for Old Tampa Bay to address recurring water quality issues in that bay segment were presented to the Tampa Bay Nitrogen Management Consortium (TBNMC) and Old Tampa Bay working group (OTBWG) in 2025 (Stantec Consulting Services, Inc. 2024, 2025a, 2025b).

Key findings from the Old Tampa Bay Assimilative Capacity Assessment project include: 1) Relationships among nitrogen, chlorophyll, water clarity, and seagrass within OTB vary over time and space within OTB sub-segments, and now differ from the broader-scale relationships documented in Tampa Bay as a whole; 2) Although linear pathways between nitrogen, chlorophyll, light attenuation, and seagrass remain valid within OTB, the existing paradigm is likely too simplistic to account for potentially synergistic or compounding stressors now being observed in the bay segment; 3) While varying hydrologic inputs and resulting residence times still appear influential to chlorophyll-a NNC attainment, the nitrogen delivery ratios for Old Tampa Bay are consistently discordant with chlorophyll-a NNC attainment; 4) Absolute TN loading assessments were more robustly linked to annual chlorophyll-a NNC attainment compared to normalized loads and alternative hydrologic normalization processes did not produce more robust links to chlorophyll-a attainment; 5) Assessing attainment of seasonal chlorophyll-a management targets for the NW and CW sub-segments of OTB may provide resource managers an earlier warning sign and more proactive opportunity to adaptively manage any subsequent chlorophyll-a NNC exceedance at the annual time-scale; 6) In addition to permitted load allocations, alternative management interventions such as oyster reef restoration or reductions in managed freshwater releases from Lake Tarpon may support better attainment of water quality standards in OTB; and 7) CASM-OTB model results corroborated empirical analyses from Task 2 and other mechanistic modeling efforts (i.e., Integrated Ecosystem Model, WASP) that suggested more consistent attainment of the chlorophyll-a NNC relative to an absolute TN loading threshold of 486 tons/yr. These findings and continued reporting on completed, ongoing, and planned nitrogen load reduction projects will be incorporated into the next RA update.

Baywide coverage of seagrass is estimated to be 31,563 acres. The next SWFWMD seagrass coverage estimate will be developed from aerial photographs acquired over the winter 2025-2026 period. Between 2022 and 2024, gains were observed in all bay segments, excluding Old Tampa Bay. Annual transect surveys conducted by TBEP and its partners (shown in the seagrass transect dashboard) are used to corroborate areal trends. In 2025, slight gains in frequency occurrence in Old Tampa Bay suggest seagrasses may have increased in the last year. Additional research is being pursued by the Southwest Florida Seagrass Working Group to better understand the potential influence of long-term temperature increases and salinity reductions on seagrass declines (Beck et al. 2024).

Detailed results for the 2022-2026 RA implementation period are provided in Tables 3.2, 3.3, 3.4, 3.5, and 3.6 for each bay segment. Results for the Remainder Lower Tampa Bay segment (Boca Ciega Bay South, Terra Ceia Bay, Manatee River) are also included in this annual assessment (reporting began with the 2023 annual assessment). As of the 2025 reporting period, NMC Actions 2-5 are not necessary based upon observed water quality conditions within Tampa Bay, though work is being continued by the TBEP and TBNMC to further understand drivers of seagrass change and Old Tampa Bay’s current assimilative capacity. Individual annual reports of the bay’s conditions from 2025 can be found on the TBEP website, as specified in the following link (Beck, Burke, and Sherwood 2025) and the water quality dashboard. A summary of historic attainment of the regulatory chlorophyll-a thresholds for each of the bay segments is depicted in Figure 3.5.

Lastly, annual hydrologic conditions among all the major bay segments in 2025 were below 1992-1994 levels for all segments except Old Tampa Bay (Table 3.1). Therefore, hydrologic adjustments for evaluating compliance with individual entity load allocations/permitting targets should be applied for each major segment except Old Tampa Bay (Janicki Environmental, Inc. 2012, 2016). The estimated hydrologic loads for each bay segment relative to observed 1992-1994 levels are indicated in the table below. The estimated compliance load adjustment factors (if applicable) are also specified. A tool to calculate the hydrologic estimates and adjustment factors by bay segment is available online through an interactive dashboard and automatically updated as provisional hydrologic estimates are approved by monitoring agencies (https://shiny.tbep.org/tbnmc_hydrologic_estimates/).

Table 3.1:

Hydrologic load estimates in 2025 relative to 1992-1994 levels and estimated compliance load adjustment factors for the major bay segments.

Bay Segment	1992 - 1994 Hydrology (95% Prediction Interval, million m3)	Hydrology Estimate (million m3)	Compliance Load Adjustment Factor
Old Tampa Bay	383 - 548	471.32
Hillsborough Bay	753-1110	681.15	0.75
Middle Tampa Bay	524-756	490.63	0.76
Lower Tampa Bay	312-402	298.00	0.83

Figure 3.1: Historic chlorophyll-a annual averages for the four major bay segments of Tampa Bay and those that include the Remainder Lower Tampa Bay segment (Boca Ciega Bay South, Terra Ceia Bay, Manatee River). Annual averages in 2025 were below the regulatory thresholds developed under the Tampa Nitrogen Management Consortium’s nutrient management strategy in all bay segments. Vertical grey bars indicate the portion of the 2022-2026 Reasonable Assurance compliance assessment period covered by the results.

Figure 3.2: Map depicting individual station chlorophyll-a exceedences in Tampa Bay relative to FDEP regulatory thresholds for chlorophyll-a in 2025. Note individual station exceedences do not indicate failed compliance at the bay segment scale.

Figure 3.3: 2025 monthly chlorophyll-a bay segment means (red dots) compared to monthly distributions from prior years (box plots and black dots). Prior years extend to 1975 for Old Tampa Bay, Hillsborough Bay, Middle Tampa Bay, and Lower Tampa Bay, 1991 for Boca Ciega Bay South, 1989 for Terra Ceia Bay, and 1990 for Manatee River. Note that Pinellas and Manatee County data are used for Boca Ciega Bay South, Terra Ceia Bay, and Manatee River and has less frequent sampling intervals than data from the Environmental Protection Commission of Hillsborough County used for the other bay segments. Boxes encompass the 25th and 75th percentiles, while whiskers bound the interquartile range. Dots beyond the whiskers represent outliers throughout the 1975-2024 sample period.

Figure 3.4: Historic seagrass coverage estimates for Tampa Bay. The target coverage of 38,000 acres was changed to 40,000 acres in 2020 to reflect programmatic goals in the 2020 Habitat Master Plan Update (TBEP #07-20). Data source: TBEP & SWFWMD.

Figure 3.5: Attainment of adopted chlorophyll-a thresholds (1975 - 2025) in the four major bay segments and Remainder Lower Tampa Bay segment (Boca Ciega Bay South, Terra Ceia Bay, Manatee River). Green (yes) indicates that average annual chlorophyll-a thresholds were met; red (no) indicates that threshold levels were not met. Grey line is the beginning of the current Reasonable Assurance implementation period.

Table 3.2:

Demonstration of reasonable assurance assessment steps for Old Tampa Bay. Green and red squares indicate outcomes of decision points outlined in the Consortium’s reasonable assurance assessment framework.

Bay Segment Reasonable Assurance Assessment Steps	DATA USED TO ASSESS ANNUAL REASONABLE ASSURANCE					OUTCOME
	Year 1 (2022)	Year 2 (2023)	Year 3 (2024)	Year 4 (2025)	Year 5 (2026)
NMC Action 1: Determine if observed chlorophyll-a exceeds FDEP threshold of 9.3 ug/L	No (7.1)	No (6.2)	No (8.8)	No (6.6)		All years below threshold so far, not necessary for NMC Actions 2-5
NMC Action 2: Determine if any observed chlorophyll-a exceedences occurred for 2 consecutive years	No	No	No	No		All years met threshold, not necessary for NMC Actions 3-5
NMC Action 3: Determine if observed hydrologically-normalized total load exceeds federally-recognized TMDL of 486 tons/year	N/A	N/A	N/A	N/A		Not necessary due to observed water quality and seagrass conditions in the bay segment
NMC Actions 4-5: Determine if any entity/source/facility specific exceedences of 5-yr average allocation occurred during implementation period						Not necessary when chlorophyll-a threshold met

Table 3.3:

Demonstration of reasonable assurance assessment steps for Hillsborough Bay. Green and red squares indicate outcomes of decision points outlined in the Consortium’s reasonable assurance assessment framework.

Bay Segment Reasonable Assurance Assessment Steps	DATA USED TO ASSESS ANNUAL REASONABLE ASSURANCE					OUTCOME
	Year 1 (2022)	Year 2 (2023)	Year 3 (2024)	Year 4 (2025)	Year 5 (2026)
NMC Action 1: Determine if observed chlorophyll-a exceeds FDEP threshold of 15 ug/L	No (8.9)	No (6.9)	No (10.7)	No (10)		All years below threshold so far, not necessary for NMC Actions 2-5
NMC Action 2: Determine if any observed chlorophyll-a exceedences occurred for 2 consecutive years	No	No	No	No		All years met threshold, not necessary for NMC Actions 3-5
NMC Action 3: Determine if observed hydrologically-normalized total load exceeds federally-recognized TMDL of 1451 tons/year	N/A	N/A	N/A	N/A		Not necessary due to observed water quality and seagrass conditions in the bay segment
NMC Actions 4-5: Determine if any entity/source/facility specific exceedences of 5-yr average allocation occurred during implementation period						Not necessary when chlorophyll-a threshold met

Table 3.4:

Demonstration of reasonable assurance assessment steps for Middle Tampa Bay. Green and red squares indicate outcomes of decision points outlined in the Consortium’s reasonable assurance assessment framework.

Bay Segment Reasonable Assurance Assessment Steps	DATA USED TO ASSESS ANNUAL REASONABLE ASSURANCE					OUTCOME
	Year 1 (2022)	Year 2 (2023)	Year 3 (2024)	Year 4 (2025)	Year 5 (2026)
NMC Action 1: Determine if observed chlorophyll-a exceeds FDEP threshold of 8.5 ug/L	No (5)	No (3.7)	No (7.8)	No (5.5)		All years below threshold so far, not necessary for NMC Actions 2-5
NMC Action 2: Determine if any observed chlorophyll-a exceedences occurred for 2 consecutive years	No	No	No	No		All years met threshold, not necessary for NMC Actions 3-5
NMC Action 3: Determine if observed hydrologically-normalized total load exceeds federally-recognized TMDL of 799 tons/year	N/A	N/A	N/A	N/A		Not necessary due to observed water quality and seagrass conditions in the bay segment
NMC Actions 4-5: Determine if any entity/source/facility specific exceedences of 5-yr average allocation occurred during implementation period						Not necessary when chlorophyll-a threshold met

Table 3.5:

Demonstration of reasonable assurance assessment steps for Lower Tampa Bay. Green and red squares indicate outcomes of decision points outlined in the Consortium’s reasonable assurance assessment framework.

Bay Segment Reasonable Assurance Assessment Steps	DATA USED TO ASSESS ANNUAL REASONABLE ASSURANCE					OUTCOME
	Year 1 (2022)	Year 2 (2023)	Year 3 (2024)	Year 4 (2025)	Year 5 (2026)
NMC Action 1: Determine if observed chlorophyll-a exceeds FDEP threshold of 5.1 ug/L	No (3.6)	No (2.6)	No (4.6)	No (3)		All years below threshold so far, not necessary for NMC Actions 2-5
NMC Action 2: Determine if any observed chlorophyll-a exceedences occurred for 2 consecutive years	No	No	No	No		All years met threshold, not necessary for NMC Actions 3-5
NMC Action 3: Determine if observed hydrologically-normalized total load exceeds federally-recognized TMDL of 349 tons/year	N/A	N/A	N/A	N/A		Not necessary due to observed water quality and seagrass conditions in the bay segment
NMC Actions 4-5: Determine if any entity/source/facility specific exceedences of 5-yr average allocation occurred during implementation period						Not necessary when chlorophyll-a threshold met

Table 3.6:

Demonstration of reasonable assurance assessment steps for Remainder Lower Tampa Bay. Green and red squares indicate outcomes of decision points outlined in the Consortium’s reasonable assurance assessment framework.

Bay Segment Reasonable Assurance Assessment Steps	DATA USED TO ASSESS ANNUAL REASONABLE ASSURANCE					OUTCOME
	Year 1 (2022)	Year 2 (2023)	Year 3 (2024)	Year 4 (2025)	Year 5 (2026)
NMC Action 1: Determine if observed chlorophyll-a exceeds FDEP threshold of 5.1 ug/L	No (3.6)	No (2.6)	No (4.6)	No (3)		All years below threshold so far, not necessary for NMC Actions 2-5
NMC Action 2: Determine if any observed chlorophyll-a exceedences occurred for 2 consecutive years	No	No	No	No		All years met threshold, not necessary for NMC Actions 3-5
NMC Action 3: Determine if observed hydrologically-normalized total load exceeds federally-recognized TMDL of 349 tons/year	N/A	N/A	N/A	N/A		Not necessary due to observed water quality and seagrass conditions in the bay segment
NMC Actions 4-5: Determine if any entity/source/facility specific exceedences of 5-yr average allocation occurred during implementation period						Not necessary when chlorophyll-a threshold met

Beck, M. W., M. C. Burke, and E. T. Sherwood. 2025. “2024 Tampa Bay Water Quality Assessment.” 03-25. St. Petersburg, Florida: Tampa Bay Estuary Program. https://drive.google.com/file/d/1jZwjt6c0LdiGL3Arl6uFVIn25Iji4ITx/view?usp=sharing.

Beck, M. W., K. E. Flaherty-Walia, S. Scolaro, M. C. Burke, B. T. Furman, D. J. Karlen, C. Pratt, C. J. Anastasiou, and E. T. Sherwood. 2024. “Hot and Fresh: Evidence of Climate-Related Suboptimal Water Conditions for Seagrass in a Large Gulf Coast Estuary.” Estuaries & Coasts 47: 1475–97. https://doi.org/https://doi.org/10.1007/s12237-024-01385-0.

Janicki Environmental, Inc. 2012. “Development of a Screening Level Tool for Estimating Annual Hydrologic Loadings to Tampa Bay.” 05-12. St. Petersburg, Florida: Tampa Bay Estuary Program. https://drive.google.com/file/d/1C6Arwat9IxYs8jeTZpcmBB6WBTpeQ2nw/view?usp=drivesdk.

———. 2016. “Update on the Development of a Screening Level Tool for Estimating Annual Hydrologic Loadings to Tampa Bay.” 03-16. St. Petersburg, Florida: Tampa Bay Estuary Program. https://drive.google.com/file/d/11NT0NQ2WbPO6pVZaD7P7Z6qjcwO1jxHw/view?usp=drivesdk.

Stantec Consulting Services, Inc. 2024. “Old Tampa Bay (OTB) Assimilative Capacity Assessment Project: Task 1 Evaluating Existing Management Paradigm and Assessing Alternatives.” 09-24. St. Petersburg, Florida: Tampa Bay Estuary Program. https://drive.google.com/file/d/1IfVUTLE20wzVgzULuyeRGWapEFT3DhQ0/view?usp=drive_link.

———. 2025a. “Old Tampa Bay Assimilative Capacity Study: Task 2 Evaluation of Eutrophication Indicator Targets, Thresholds, and Numeric Nutrient Criteria and Proposed Adjustments.” 05-25. St. Petersburg, Florida: Tampa Bay Estuary Program. https://drive.google.com/file/d/12xYfSs-4bxhLa1ZVQbmhQSv5Jo4Jf-oY/view?usp=sharing.

———. 2025b. “Old Tampa Bay Assimilative Capacity Study: Task 3 Management Intervention Assessment and Recommendations.” 17-25. St. Petersburg, Florida: Tampa Bay Estuary Program. https://drive.google.com/file/d/1uVeMIpSylz1Mull6WMwVDc7KVInzosqy/view?usp=sharing.