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coastal waters, including Biscayne Bay. FIU has been monitoring WQ in South Florida
since the early 1990’s, hence, expanding the objectives to incorporate all estuaries and
coastal waters from North Biscayne Bay, to Pine Island Sound, on the western coast
(Fig 6.1) was a natural progression to other tasks in the project.
Among the several reasons for on-going water quality problems in South Florida
is the unenforceability of its narrative water quality criteria. Existing Outstanding Florida
Waters, anti-degradation, standards and many of the Class III, protection of designated
use, standards are non-numeric, making their applicability very limited at best. Thus,
there is an immediate need to change their narrative format to a more practical and
usable numeric format through the implementation of valid processes and sound
statistical methods. The US Environmental Protection Agency (USEPA) and the Florida
Department of Environmental Protection (FDEP) are in the process of deriving such
numeric nutrient criteria for South Florida estuaries and coastal waters, which must be
promulgated by late 2012.
Most of these waters are either within designated National Parks lands or within
their influence zones, and considering that it is responsibility of the National Park
Service to protect and preserve the natural and cultural resources and values of the
National Park System, NPS has a great interest in EPA’s and FDEP’s efforts for
developing t numeric surface water quality criteria for key nutrients.
Water quality of South Florida’s estuaries and coasts is the result of a long-term
and poorly understood interplay of local, regional and global forcing, drivers, pressures
and responses. Monitoring programs render water quality (WQ) snapshots taken during
the last 20 years from which researchers attempt to produce not only a motion picture of
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the past, but a projection of future scenarios. South Florida’s coastal and estuarine
waters have experienced the impact of anthropogenic interventions since the early
1900’s, including major disruptions of its hydrology and also sustained urban and
agricultural development (Davis and Ogden, 1994; Hunt et al. 2007; Nuttle et al. 2000;
RECOVER 2005). Furthermore South Florida (SoFlo) waters are influenced by distant
sources, such as the Gulf of Mexico and the Mississippi River. Hence, SoFlo aquatic
ecosystems bear the heritage and signals of these long and sustained influences.
Figure 6.1: Spatial coverage of FIU Water Quality Monitoring Network.
(http://serc.fiu.edu/wqmnetwork/).
Given the ecological impacts caused by water management of the Everglades
since the last century, the probabilities of finding a pristine water body are meager.
Nevertheless, most South Florida estuaries are oligotrophic and practically algal bloomfree, except for some restricted areas (Central Florida Bay and North Biscayne Bay)
where occurrence of algal blooms is chronic. The USEPA recommends three types of
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approaches for setting numeric nutrient criteria, including: reference condition
approaches, stressor-response analysis, and mechanistic modeling. This study will
apply the second of these approaches and use FIU’s water quality monitoring data to
describe stressor-response relationships and use this information to derive numeric
nutrient criteria. We will first focus our efforts on defining baseline conditions. We will
develop water quality targets using information on the relationship between Total
Nitrogen (TN) and Total Phosphorous (TP) as causal enrichment variables, and
Chlorophyll a (CHLa) as initial response variable, given its recognized value as
ecological indicator of nutrient enrichment (Boyer et al. 2009).
SEGMENTATION OF SOUTH FLORIDA WATERS
It has been well documented that South Florida estuaries have different water
quality characteristics due to differences in geomorphology, water circulation, residence
time, soil type in their watershed and bay bottom, benthic communities, and
management practices. We evaluated these characteristics and reassessed all
previously established subdivisions for Florida Bay, Ten Thousand Islands, Whitewater
Bay, Pine Island Sound-Rookery Bay, Florida Keys and Biscayne Bay. After an initial
stage of QA/QC, we redefined the Period of Record (POR) for each basin depending
upon data availability and variable set completeness for the whole set of
biogeochemical parameters (total and dissolved nutrients, CHLa, turbidity, temperature,
salinity, DO and light extinction). We then calculated descriptive statistics and
performed long-term trend exploration of the redefined POR time-series, to gain insight
into patterns of behavior along the POR for all relevant biogeochemical parameters.
This exploration was performed with Akritas Thiel-Sen slope calculations and z-scored
cumulative sum charts (explained above).
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Figure 6.2: Flow diagram illustrating the steps followed in the derivation of Numeric
Nutrient Criteria as applied to South Florida coastal and estuarine waters.
A well recognized spatial-temporal variability within estuaries and coastal waters
is present in our individual SoFlo basins. This fact called for a holistic approach to basin
segmentation to account for variability not only dictated by a given nutrient
concentration level, but by the combination of imposed conditions such as: nutrients,
water clarity, climate, weather, extreme events, geomorphology, circulation and
exchange, and management. As done for Biscayne Bay, basin segmentation was
accomplished with an objective classification of station sites combining Principal
Component Analysis (PCA) and Hierarchical Clustering methods in tandem. Selected
biogeochemical variables (8 to 13) for each of the five basins (Florida Bay, Florida Keys,
Whitewater Bay-10,000 Islands, Gulf Shelf and Pine Island-Rookery Bay) were used for
PCA. Statistics (mean, standard deviation, median and median absolute deviation) of
retained scores from PCA were input into Hierarchical clustering routines. The rationale
behind the selection of these statistics was to account not only for level of individual
parameter but also for their variability. Then, progressive subdivisions were obtained
varying the statistical distance among groups in the cluster tree. Selection of the final
number of segments was subjective and knowledge-based. Spatial extension and
pattern, geomorphology, water circulation and benthic ecosystem distribution played a
major role on the decision. In summary, we statistically characterized and subdivided
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waters in each basin into what we considered biogeochemically and spatially coherent
segments.
We identified 40 water-types for SoFlo coastal and estuarine waters that extend
from Biscayne Bay in the east to Dry Tortugas in the southwest and to Pine Island
Sound in the northwest (Table 6.1, Fig 6.3). Segment maps were generated placing the
separating lines approximately midway in between clustered stations, or followed the
FATHOM Model subdivision (Cosby et al. 2005), followed previous subdivisions or were
arbitrarily drawn following general geomorphologic patterns. In summary, border lines
are site-specific and not the result of systematic spatial statistical analysis.
Non-parametric Mann-Whitney and Kruskal-Wallis tests were used to compare
biogeochemical variables among segments, and box-and-whisker plots to summarize
descriptive statistics. The former analyses highlighted statistically significant differences
among segments, and the later underscored anomalous and probably impacted stations
and segments. Sustained high concentration levels would suggest sustained impacted
and perhaps impaired conditions – not meeting designated use type. Anomalous
stations isolated from permanent sources of human disturbance (urban areas, canal