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basin. It is difficult to identify one dominant cause of disturbance in habitat over the |
course of the sampling period due to the limitations of the study and the number of |
possible variables involved in vegetation growth and distribution. |
4.4.1 Algae and Nutrient Changes Related to Seagrass Variations |
During the past century, human activities have had a tremendous impact on the |
global cycling of nutrients in coastal systems (Caccia and Boyer 2007). The export of |
phosphorus to the oceans has increased threefold compared to pre-industrial and preagricultural levels while the export of nitrogen has increased even more dramatically |
(Caraco 1995). Most seagrass beds contain benthic and epiphytic algae (FMNH 2015) |
that may contribute significantly to the system (Heijs 1987; Verheij and Erftemeijer |
1993; Jupp et al. 1996; Sidik et al. 2001). However, when a water quality disturbance |
involving nutrients occurs in an area, there is the potential to shift the dynamics between |
the seagrass and algae cover (Fourqurean et al. 1995; FMNH 2015). Multer (1988) found |
that high biomass of macroalgae appears under the conditions on low and moderate |
seagrass shoot density, indirectly demonstrating the competitive relationship between |
seagrasses and macroalgae. It was expected that there would be stronger competition |
between grass and algae in POM because of the low seagrass cover-density recorded; |
however, no significant shifts were detected during the study period. The lowest |
79 |
occurrence of algae in record in POM was in 2006 and the highest in 2007 (Figure 5). |
Without an accurate record of nutrient concentrations for these sample years, it is difficult |
to determine the cause behind this shift. It is possible the change in occurrence may be |
due to overall shallower sites sampled in 2007, which may support more algae species. |
Monitoring over the past century has been inconsistent in the POM region, so it is |
difficult to establish a solid data record. The useable data from the SFWMD agencies |
showed little to no significant variation in measured nutrients between sampling years for |
most of the variables. There were no consistent data records from the SFWMD prior to |
2008 for TP (total phosphate) or NOx (nitrate+nitrite) concentrations within the POM |
basin. Data show that available nutrient concentrations have remained fairly low over the |
last four years of the study period. Between 2008 and 2011 the TP averaged 0.004 ± |
0.017 mg/L and NOx concentrations averaged 0.004 ± 0.017 mg/L (Table 11). There are |
many forms of Nitrogen in an aquatic environment, and since the SFWMD records for |
Nitrogen measurements covered several different types between agencies, it is difficult to |
determine any significant relationships between variables within the basin. The lack of |
consistent nutrient data for some variables makes it difficult to make solid connections |
between agencies. Even with a large uncertainty in pairing the sample collection dates |
and times, is it suggested a reasonable correlation can be made for some variables |
between other agencies and monitoring programs (RECOVER 2014). |
There have been indications that the Biscayne Bay ecosystem is slowly |
recovering, although conditions are not expected to return to those of the early 1900s, |
before settlement (Cantillo et al. 2000). The water quality in North Biscayne Bay has |
improved substantially in the last 30 years and now generally meets or exceeds local, |
state and federal standards for recreational uses and propagation of fish and wildlife |
through regulatory action and shoreline revetment and restoration projects (SFWMD |
2000). DERM has even documented significant return of benthic communities in some |
portions of North Biscayne Bay as a result of improved water quality (SFWMD 2000). |
The bay still receives a considerable amount of nutrients, trace metals, organic chemicals |
and particulates from storm water runoff, canal discharge, and other sources (BBAP |
2011) which is still a concern for environmental agencies. |
80 |
Initially detected in 2002, a macroalgae bloom composed of mainly Anadyomene |
spp. has continually persisted along parts of the western shoreline of Biscayne Bay, just |
south of the Rickenbacker Causeway and the Port of Miami basin (RECOVER 2014). |
The impact of the prolonged algae bloom has resulted in a shift from a seagrass |
dominated community to a macroalgal dominated community in that region (RECOVER |
2012). Macroalgae can potentially have one of the greatest effects on the seagrass coverdensities in an area, but even with some significant increases in cover detected in of some |
algae groups in the POM between years, there was no obvious detrimental impact |
observed to the seagrass. The fragile balance between the seagrass and algae must be |
maintained through proper water management practices. |
The Integrated Biscayne Bay Ecological Assessment and Monitoring Project |
(IBBEAM) report for CERP combined the sampling efforts of four individual projects |
funded by the RECOVER Monitoring and Assessment Plan (MAP): Salinity Monitoring |
Network, Nearshore SAV (submerged aquatic vegetation), Alongshore Epifauna, and |
Mangrove Fish. The project concluded that the nearshore habitats in Biscayne Bay are |
occupied by floral and faunal species assemblages operating below their productive |
potential, due, in part, to inadequate and unnatural freshwater flows limiting the duration |
and spatial extent of mesohaline conditions (RECOVER 2014). Despite this, IBBEAM |
determined that the occurrence of H. wrightii and T. testudinum in the study area |
remained largely stable over the period of record, similar to the findings from this study |
(FIAN). Seagrass cover-density and spatial distribution fluctuated somewhat seasonally |
and varied across years within the POM basin, but no clear temporal trend in seagrass |
cover was apparent. The seagrass species remain fairly stable within the port. |
4.5 Future Threats to Seagrass Habitats |
Over the next century, the predicted changes in global climate will alter many of |
the factors that shape the coastal ecosystem of South Florida (RECOVER 2014). Climate |
change has the potential to cause sea level rise, increased temperatures, changes in |
precipitation patterns, and changes in the intensity and/or frequency of extreme weather |
events (ICLEI 2010). This can lead to a rapid loss and substantial changes to the benthic |
communities along the coasts (Wanless et al. 1994). Studies done by the Organization |
for Economic Cooperation and Development (OECD) identified Miami–Dade as the |
81 |
county with the highest amount of vulnerable assets exposed to potential coastal flooding, |
with costs projected at around $3.5 trillion (Nicholls et al. 2007). A major consequence |
of climate change is an increase in water temperature in the North Atlantic. Models |
predict that increases in tropical sea surface temperature can lead to an increase in |
number and intensity of tropical storms, with larger peak wind speeds and more heavy |
precipitation (Mann et al. 2009; IPCC 2007). Global sea surface temperature increasing |
by one degree Celsius could result in a 30% increase in category 4 and 5 storms |
worldwide (Elsner et al. 2008). Over the past decade the Copenhagen Diagnosis (a report |
written by twenty-six climate scientist from eight countries) has found evidence of a |
global increase in the number of category 4 and 5 hurricanes (Allison et al. 2009). These |
climate deviations can alter habitat productivity and eutrophication by changing the |
delivery of fresh water, nutrients, and sediments (Scavia et al. 2002). With climate |
changes, the composition and distribution of biotics that occur in the coastal ecosystem |
will shift (Scavia et al. 2002). |
5.0 Conclusions |
The primary objectives of this study were to: 1) characterize the seagrass |
community (e.g., species composition, cover, and distribution); and 2) characterize the |
environmental and physical conditions (e.g., salinity, temperature, turbidity, and water |
and sediment depths); and 3) determine if there are relationships between the seagrass |
community and the environmental and physical conditions; and 4) evaluate if the natural |
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