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patrickbdevaney
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mia9

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the Florida Keys reef tract after the 1980’s; and sustained nutrient concentration decline
in waters leaving the Everglades (Briceño and Boyer 2007, 2008).
Recent changes in BB have been reported by Migliaccio et al. (2008) who
studied biogeochemical trends from years 1996 to 2007, using DERM, NPS and FIU
data. They reported increasing trend for chlorophyll-a likely responding to the increase
in NH4-N, SRP, and TPO4-P, in turn responding to a dramatic population increase and
land use change in the watershed from agricultural land to residential and urbanizing
areas.
Our analysis of time-series components was performed with Akritas Theil/Sen
regression (ATS) for secular trend (Appendix 9) and Zcusum charts for detailed
scrutiny. A Cusum chart is a plot of the cumulative sum of deviations (Sn= Σ[zi-T] for
i=1….n) from a target (T), in our case, the time-series grand mean, against n, the
sample number (or date if regularly sampled) (Ewan, 1963). Cusum charts and Cusum
analysis are standard procedure in the field of industrial process control (Duncan 1974;
Grant and Leavenworth 1980; Montgomery 2001). For a detailed explanation on
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construction, properties and applications of Zcusum charts, the reader is referred to
Appendix 4). Visual inspection of Zcusum charts gives a detailed history of the
parameters variability along the time-line or another variable gradient providing more
information than ATS, ordinary linear regression or Line of Organic Correlation.
Finally, we must always keep in mind that trend analysis is limited to the window
of observation; trends change with continued data collection. In addition, water quality in
South Florida may fluctuate according to disturbance events of longer periodicity, and
trends may even reverse during a given period of record.
Total Nitrogen
As shown in Fig 3.19, TN declined across the Bay displaying two cycles common
to all segments during the POC. From 1996 to 1999 values were in general above
average (positive Zcusum slope), followed by a decline to below average values until
2002, except in SCO where the decline began in 1998. The second cycle extends from
2003 to 2007, with the break from above to below average approximately occurring at
the end of 2004. In MBS the second cycle is less developed. Finally, seasonality is
crudely developed, perhaps reflecting managed water deliveries non tuned to natural
rain cycles and water inflow.
Total Phosphorous
Phosphorous displays a common pattern to all segments in Biscayne Bay, except MBS,
consisting of (Fig 3.20):
 Below average TP values until the end of 1998, followed by above average TP to
early-mid 2001
 Seasonal changes with below average values until early 2002 followed by above
average TP to late 2002
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Figure 3.19: TN Zcusum charts for water quality segments in Biscayne Bay
 Below average TP until early 2005 followed by a long above average period,
which started with a sudden increase in concentration in the summer 2005
(Wilma impact), followed by a less steep decline until mid-to-late 2006 when
again a sudden increase in TP occurred.
 Finally, TP declines from early 2007 to the end of the POC.
The extreme impact of hurricanes Katrina, Rita and especially Wilma in 2005, distorted
the TP pattern at MBS (Fig 3.20) which otherwise would follow a similar pattern as the
rest of the Bay. In this context, TP across all Biscayne Bay was declining until 2005. As
for TN, seasonality is not well defined for TP either.
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