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patrickbdevaney
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mia9

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Driver
Time
Response
Time
Response
Driver
Response Z-Cusum
Driver
a) Driver threshold b) Response threshold c) Driver-Response hysteresis
Modified after Andersen et al 2008
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The relative location of the median nutrient concentration within the Zcusum
chart and its relation to the threshold position deserve special consideration. As shown
in Figure 6.5 median TN and median TP are above the CHLa thresholds, suggesting the
initiation of bloom conditions below median nutrient levels in the water column.
Furthermore, in all instances the median nutrient concentrations fall on the aboveaverage branch of the CHLa Zcusum chart (positive slope branch), indicating that
median nutrient concentrations are associated to above average CHLa production.
Finally, relationships for the overall region show that median TN and median TP are
above the CHLa thresholds almost everywhere. Those relationships suggest the
initiation of bloom conditions below median nutrient levels in the water column. Bloom
conditions simply means significant increase in biomass (Legendre 1990), abundance
(Lapointe 1999), or population size (Smyda 1997; Carstensen et al 2004; Sparrow
2007). In our case, the beginning of bloom conditions indicated by the threshold
correspond to the first sudden, significant and sustained CHLa increase.
Criteria
Although coastal and estuarine waters in South Florida have been affected by
human intervention and nutrient enrichment (Davis and Ogden 1994) most waterbodies
remain under oligotrophic-mesotrophic conditions, and most seem to meet with their
designated use. These characteristics have lead regulators to suggest numeric criteria
designed to maintain the current data distribution hoping to protect those uses in the
future (FDEP 2011). Our long-term trend analysis indicate that there are periods of
nutrient enrichment and associated water quality deterioration (i.e. algal blooms) which
may be linked to management practices, disturbances and/or climate variability. Hence,
statistics derived from the overall dataset, which includes these ―anomalous‖ periods,
will be biased towards above ―baseline‖ conditions.
Defining baseline conditions for each station or segment is a complicated task
given the specific characteristics of each site, but we have determined important TN and
TP concentration thresholds separating what seems to represent nutrient baseline
conditions from those conditions that may foster significantly increased phytoplankton
biomass (CHL-a) in the water column. Such increases (blooms) may result from
93
seasonal nutrient enrichment or from environmental disturbances in general, so the
threshold may be envisioned as the mean baseline condition to guarantee a protective
nutrient level in the long run.
Figure 6.5: Chlorophyll-a Zcusum charts along TN and TP gradients for a West-East transect
in South-central Biscayne Bay. ―V-shape‖ charts indicate increasing CHLa concentration and
―peak-shape‖ charts indicate declining trend. Red, blue and yellow vertical markers are located
at threshold, median and 75th percentile of nutrient concentrations, respectively.
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
60
0.000
0.001
0.010
0.100
1.000
CHLa Z-Cusum (SD) .
TP (mg/l)
OFF CHLa CT Median Z Threshold Median TP 75pctl TP
-35
-30
-25
-20
-15
-10
-5
0 0.01
0.10
1.00
10.00
CHLa Z-Cusum
TN (mg/l)
SCI CHLa Central Tendency Threshold Median TN 75pctl TN
-50
-40
-30
-20
-10
0
10
0.01
0.10
1.00
10.00
CHLa Z-Cusum
TN (mg/l)
SCM CHLa Central Tendency Threshold Median TN 75pctl TN
-90
-80
-70
-60