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### Long-term trends in the water quality of Florida Bay (1989 - 2000) Joseph N. Boyer & Ronald D. Jones Southeast Environmental Research Center, Florida International University, Miami, FL 33199 One of the primary purposes for conducting long-term monitoring projects is to be able to detect trends in the measured variables over time. These programs are usually initiated as aresponse to public perception (and possibly some scientific data) that 'the river-bay-prairie-forestetc. is dying'. In the case of Florida Bay during 1987, the impetus was the combination of a seagrass die-off, increased phytoplankton abundance, sponge mortality, and a perceived decline in fisheries. In response to these phenomena, a network of water quality monitoring stations was established in 1989 to explicate both spatial patterns and temporal trends in water quality in an effort to elucidate mechanisms behind the recent ecological change. Overall Period of Record A spatial analysis of data from our monitoring program resulted in the delineation of 3 groups of stations which have robust similarities in water quality (Fig. 1). We have argued that these spatially contiguous groups of stations are the result of similar loading and processing of materials, hence we call them 'zones of similar influence'. The Eastern Bay zone acts most like a 'conventional' estuary in that it has a quasi-longitudinal salinity gradient caused by the mixing of freshwater runoff with seawater. In contrast, the Central Bay is a hydrographically isolated area with low and infrequent terrestrial freshwater input, a long water residence time, and high evaporative potential. The Western Bay zone is the most influenced by the Gulf of Mexico tides and is also isolated from direct overland freshwater sources. Climactic changes occurring over the data collection period of record had major effects on the health of the bay. Precipitation rebounded from the drought during the late 80's being greater than the long term average (9.2 cm mo-1) for the last 7 of 10 years (Fig 2.). Over this period, salinity and total phosphorus (TP) concentrations declined baywide while turbidity (cloudiness of the water) increased dramatically. The salinity decline in Eastern, Central and Western Florida Bay was 13.6, 11.6, and 5.6 psu, respectively (Fig. 3). Some of this decrease in Eastern Bay could be accounted for by increased freshwater flows from the Everglades but declines in other areas point to the climactic effect of increased rainfall during this period. The Central Bay continues to experience hypersaline conditions (>35 psu) during the summer but the extent and duration of the events is much smaller. As mentioned previously, TP concentrations have declined baywide over the 10 year period (Fig. 4). The Eastern Bay has the lowest concentrations while the Central Bay is highest. Unlike most other estuaries, increased terrestrial runoff may have been partially responsible for the decrease in TP concentrations in the Eastern Bay. This is because the TP concentrations of the runoff are at or below ambient levels in the bay. The elevated TP in the Central Bay is mostly due to concentration effect of high evaporation. It is important to understand that almost all the phosphorus measured as TP is in the form of organic matter which is less accessible to plants and algae than inorganic phosphate. Turbidity in Eastern Bay increased 2-fold from 1991-98, while Central and Western Bays increased by factors of 20 and 4, respectively (Fig. 5). Generally, the Eastern Bay has the clearest5 water which is due to a combination of factors such as high seagrass cover, more protected basins, low tidal energy, and shallow sediment coverage. Turbidity in the Central and Western Bays have increased tremendously since 1991. We are unsure as to the cause but the loss of seagrass coverage may have destabilized the bottom so that it is more easily disturbed by wind events. Chlorophyll a concentrations (Chl a), a proxy for phytoplankton biomass, were particularly dynamic and spatially heterogeneous (Fig. 6). In the Eastern Bay, which makes up roughly half of the surface area of Florida Bay, Chl a declined by 0.9 μg l -1 or 63%. The isolated Central Bay zone underwent a 5-fold increase in Chl a from 1989-94, then rapidly declined to previous levels by 1996. In Western Florida Bay, there was a significant increase in chlorophyll a, yet median concentrations of chlorophyll a in the water column remained modest (~2 μg l -1 ) by most estuarine standards. There were significant blooms in Central and Western Bays immediately following Hurricanes Georges (Nov. 1998) and Irene (Oct. 1999). It is important to note thatthese changes in turbidity and chlorophyll a happened after the poorly-understood seagrass dieoff in 1987. It is likely that the death and decomposition of large amounts of seagrass biomass can at least partially explain some of the changes in water quality of Florida Bay but the connections are temporally disjoint and the processes indirect and not well understood. Ammonium (NH 4 + ) levels displayed large variability over the period of record and was much higher in the Central Bay than anywhere else (Fig. 7). Only in Central Bay did the NH 4 + pool increase substantially over time (3-6 fold). Trends in nitrate (NO 3 -2 ) concentrations mirrored those of NH 4 + and were mostly due to the biological conversion of NH 4 + to NO 3 -2 via nitrification (Fig. 8). Total organic carbon concentrations (TOC) vary widely among the different zones and show significant intra-annual cycles (Fig. 9). Highest TOC levels generally occur in the Central Bay during summer as a result of evaporative concentration and restricted mixing with the rest of the Bay. 2000 Alone Most water quality variables during 2000 generally followed typical annual trends but there were a couple exceptions. Both Central and Western Bays experienced hypersalinity during the summer months. Salinity in the Western Bay was ~ 45 psu during Sept.; the Central Bay got up to 48 psu and remained hypersaline during June – Sept. In addition, a moderate phytoplankton bloom (4-12 μg l -1 ) occurred in Central Bay during March and April. A CKNOWLEDGMENTS We thank all the field and laboratory technicians involved with this ongoing project especially Pete Lorenzo. This project was possible due to the continued funding of the South Florida Water Management District through the Everglades National Park (SFWMD/NPS Cooperative Agreement C-7919 and NPS/SERC Cooperative Agreement 5280-2-9017). This report is contribution #T XXX of the Southeast Environmental Research Center at Florida International University. Maps of South Florida Water Quality Monitoring Network http://serc.fiu.edu/wqmnetwork6 Figure 1. Zones of similar water quality in Florida Bay 

# Monthly Average Rainfall Figure 2. Monthly average rainfall in the Florida Bay area. 

### 0 10 20 30 40

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000cm mo-17 

# Median Salinity

Figure 3. Monthly median salinity in the three Florida Bay zones. 

## Eastern 0 10 20 30 40 50 60
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001psu 

## Central 10 20 30 40 50 60 70
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001psu 

## W estern 25 30 35 40 45 50 55
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001psu8 

# Median Total Phosphorus

Figure 4. Monthly median total phosphorus concentration in the three Florida Bay zones. 

## Eastern 0. 000 0. 005 0. 010 0. 015 0. 020
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## Central 0. 00 0. 01 0. 02 0. 03 0. 04 0. 05 0. 06 0. 07
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## W estern 0. 00 0. 01 0. 02 0. 03 0. 04 0. 05
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm9 

# Median Turbidity

Figure 5. Monthly median turbidity levels in the three Florida Bay zones. 

## Eastern 0 2 4 6 8 10 12 14 16 18
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001NTU 

## Central

0 10 20 30 40 50 60 70
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001NTU 

## W estern

0 10 20 30 40 50 60
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001NTU10 

# Median Chlorophyll a

Figure 6. Monthly median chlorophyll a concentrations in the three Florida Bay zones. 

## Eastern 0. 0 0. 5 1. 0 1. 5 2. 0
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001μg l-1 

## Central

0 5 10 15 20
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001μg l-1 

## W estern

0 2 4 6 8 10 12 14 16
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001μg l-111 

# Median Ammonium (NH 4 + 

# )

Figure 7. Monthly median ammonium concentrations in the three Florida Bay zones. 

## Eastern 0. 00 0. 02 0. 04 0. 06 0. 08 0. 10 0. 12 0. 14
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## Central 0. 0 0. 1 0. 2 0. 3 0. 4 0. 5
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## W estern 0. 00 0. 02 0. 04 0. 06 0. 08 0. 10
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm12 

# Median Nitrate (NO 3 -2 

# )

Figure 8. Monthly median nitrate concentrations in the three Florida Bay zones. 

## Eastern 0. 00 0. 01 0. 02 0. 03 0. 04
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## Central 0. 00 0. 01 0. 02 0. 03 0. 04 0. 05
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## W estern 0. 00 0. 01 0. 02 0. 03
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm13 

# Median Total Organic Carbon (TOC)

Figure 9. Monthly median TOC concentrations in the three Florida Bay zones. 

## Eastern 4 6 8 10 12 14 16
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## Central

0 5 10 15 20 25 30 35
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm 

## W estern

0 2 4 6 8 10 12 14
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001ppm