Step-by-Step Guide to Applying Assessment of Estuarine Trophic Status (ASSETS)
Compiled by Cayce Dalton, January-May 2006
I. Influencing Factors
Dilution Potential
Determine for the estuary:
Is there vertical stratification? (homogenous, minor, yes)
What is the freshwater inflow (m2/day)?
What is the mean volume of estuary?
Determine dilution potential (Bricker et al., 1999, p.59):
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Flushing Potential
Determine for the estuary:
Category of tidal range (macro if > 6 ft., meso if 6 – 2.5 ft, micro if <2.5 ft.)
The order of magnitude of freshwater inflow / estuary volume (100 - 10-4)
Determine flushing potential (Bricker et al., 1999, p.59):
Susceptibility
Determine susceptibility from dilution and flushing potentials (Bricker et al., 1999, p.60):
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Influencing Factors Nutrient Loading Formula
The basic quantitative model is:
Influencing Factors Nutrient Loading Formula = mh / (mb + mh)
where:
mh = DIN concentration derived from humans in watershed
mb = DIN concentration from offshore
The difficult part is getting mh and mb. You can use loads or concentrations.
Using concentrations (Bricker et al., 2003, p.46):
mh = min * (So – Se)/So
and
mb = msea * Se/So
where:
msea = DIN concentration of the ocean (end member)
Se = Salinity of estuary (avg ppt) (see Smith et al., 2003)
So = Salinity of ocean (end member)
min = DIN concentration in inflow to the estuary
See the example file “ASSETS OHI Formula Webhannet.xls”
Loadings may be used in place of the above simple model. That method is not discussed
in the ASSETS literature, so isn’t presented here.
A third way discussed by the project team to determine mh and mb was by solving a
simple system of linear equations. This method is not presented here, see end of
“ASSET Training.ppt”
Determine Influencing Factors nutrient loading (Bricker et al., 2003, p.46)
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Influencing Factors (formerly “Overall Human Influence”)
Determine Influencing Factors from OHI Formula result (above) and susceptibility
(Bricker et al, 2005, no page number). This is cited as originally from Bricker et al.
(2003), but it is not in that document. (Note also that there are 5 categories of Influencing
Factors above, but only 3 categories below.)
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II. Overall Eutrophic Condition
In this section, you determine symptom scores for each salinity zone in the estuary
separately. Then you weight these scores based on the size of the zone.
Salinity Zones
In the original source material, there are two discrepancies regarding the definition of
salinity zones the ASSETS materials. A full description is presented at the end of this
document. Below is the method as it was actually applied in our SWMP synthesis
project:
Using GIS and all available data, delineate three salinity zones for the estuary based on
annual average salinity:
Freshwater Tidal zone is <0.5 ppt
Mixing zone is 0.5 – 25 ppt
Seawater zone is >25 ppt
Using the zones above, in GIS calculate the surface areas of:
Freshwater Tidal zone
Mixing zone
Seawater zone
Total estuary
Note the total number of zones in the estuary.
Aggregating Zone Results
The above results are used to aggregate zone-specific values into one estuary-wide value
for each parameter in the next section as follows (Bricker et al., 2003, p.47):
“…an area weighted expression value for each zone was determined, and
the symptom level of expression Sl was then obtained by summation.
where Az is the surface area of each zone; Ae is the total estuarine surface area;
El is the expression value at each zone; n is the number of estuarine zones.”
For example, the areas and expression values below…
Zone
Area
Area /
Expression Weighted Score
Total Area
Value
Freshwater 25 km2
1
0.526 x 1 = 0.526
25÷475 = 0.526
Tidal zone
Mixing
150 km2 150÷475 = 0.375
0.25
0.375 x 0.25 = 0.079
zone
Seawater
300 km2 300÷470 = 0.632
0.5
0.632 x 0.5 = 0.316
zone
Total
475 km2
estuary
…gives the aggregated result of: 0.105 + 0.079 + 0.316 = 0.448
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Overall Eutrophic Condition (OEC) Schematic Overview
Table 7 of report body (Bricker et al, 2005, no page number).
Primary symptoms
Chl-a
Use these categories (Bricker et al., 1999, p.50):
Surface concentrations:
Hypereutrophic (>60 μg chl-a/l)
High (>20, ≤60 μg chl-a/l)
Medium (>5, ≤20 μg chl-a/l)
Low (>0, ≤5 μg chl-a/l)
Spatial coverage (% of salinity zone):
High (>50, ≤100%)
Medium (>25, ≤50% )
Low (>10, ≤25%)
Very Low (>0, ≤10% )
Frequency (considered as a mult-year pattern, Bricker 2005 personal communication)
Episodic (conditions occur randomly)
Periodic (conditions occur annually or predictably)
Persistent (conditions occur continually throughout the year)
Seawater zone:
Chl-a 90th percentile? (hyper, high, med, low)
Spatial coverage? (hi, mod, low, v low, unknown)
Frequency? (episodic, periodic, persistent, unknown)
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Mixing zone:
Chl-a 90th percentile? (hyper, high, med, low)
Spatial coverage? (hi, mod, low, v low, unknown)
Frequency? (episodic, periodic, persistent, unknown)
FW zone:
Chl-a 90th percentile? (hyper, high, med, low)
Spatial coverage? (hi, mod, low, v low, unknown)
Frequency? (episodic, periodic, persistent, unknown)
For each zone above, determine expression level (Bricker et al., 2003, p.48)
Aggregate the zone results into an estuary-wide result (see Aggregating Zone Results,
above).
Macroalgae
Use these categories (Bricker et al., 1999, p.50; Bricker et al., 2003, p.48)
Abundance
Problem (significant impact upon biological resources)
No problem (no significant impact)
Frequency (considered as a mult-year pattern, Bricker 2005 personal communication)
Episodic (conditions occur randomly, not necessarily every year)
Periodic (conditions occur annually or predictably)
Seawater zone:
Abundance? (problem, no problem)
Frequency? (episodic, periodic, unknown)
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Mixing zone:
Abundance? (problem, no problem)
Frequency? (episodic, periodic, unknown)
FW zone:
Abundance? (problem, no problem)
Frequency? (episodic, periodic, unknown)
For each zone above, determine expression level (Bricker et al., 2003, p.48)
Aggregate the zone results into an estuary-wide result (see Aggregating Zone Results,
above).
Secondary symptoms
DO (dissolved oxygen)
Use these categories (Bricker et al., 1999, p.50):
Surface concentrations:
Anoxia (0 mg/l)1
Hypoxia (>0mg/l ≤ 2mg/l)
Biological Stress (>2mg/l ≤ 5mg/l)
Spatial coverage (% of salinity zone):
High (>50, ≤100%)
Medium (>25, ≤50% )
Low (>10, ≤25%)
Very Low (>0, ≤10% )
Frequency (considered as a mult-year pattern, Bricker 2005 personal communication)
Episodic (conditions occur randomly)
Periodic (conditions occur annually or predictably)
Persistent (conditions occur continually throughout the year)
Seawater zone:
DO 10th percentile? (anoxic, hypoxia, bio stress, no problem)
Spatial coverage? (hi, mod, low, v low, unknown)
Frequency? (episodic, periodic, persistent, unknown)
Mixing zone:
DO 10th percentile? (anoxic, hypoxia, bio stress, no problem)
Spatial coverage? (hi, mod, low, v low, unknown)
Frequency? (episodic, periodic, persistent, unknown)
1
In the SWMP database, the DO probe has an accuracy of +/- 0.2 mg/L in the hypoxic and anoxic range.
Therefore, a level of 0.2 mg/L was considered anoxic (YSI ca. 2002).
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FW zone:
DO 10th percentile? (anoxic, hypoxia, bio stress, no problem)
Spatial coverage? (hi, mod, low, v low, unknown)
Frequency? (episodic, periodic, persistent, unknown)
For each zone above, determine expression level (Bricker et al., 2003, p.48)
Aggregate the zone results into an estuary-wide result (see Aggregating Zone Results,
above).
SAV (submerged aquatic vegetation, eelgrass):
Use these categories (Bricker et al., 1999, p.50; Bricker et al., 2003, p.48)
Trend:
Increase
Decrease
No change
Magnitude of loss:
High (>50%, ≤100%)
Medium (>25%, ≤50%)
Low (>0%, ≤25%)
Seawater zone:
Trend? (increase, decrease, no change)
Magnitude of change? (hi, med, low)
Mixing zone:
Trend? (increase, decrease, no change)
Magnitude of change? (hi, med, low)
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FW zone:
Trend? (increase, decrease, no change)
Magnitude of change? (hi, med, low)
For each zone above, determine expression level (Bricker et al., 2003, p.48)
Aggregate the zone results into an estuary-wide result (see Aggregating Zone Results,
above).
HAB (toxic algae blooms) and NTB (nuisance toxic blooms); combined into single
category (Bricker et al, 2005, p.5)
Use these categories (Bricker et al., 1999, p.50; Bricker et al., 2003,, p.48)
Presence
Problem (significant impact upon biological resources)
No problem (no significant impact)
Start offshore and advect in?
Yes
No
Duration
S = seasonal
M = months
V = variable
W = weeks
D = days
WS = weeks to seasonal
WM = weeks to months
DW = days to weeks
Frequency (considered as a mult-year pattern, Bricker 2005 personal communication)
Episodic (conditions occur randomly)
Periodic (conditions occur annually or predictably)
Persistent (conditions occur continually throughout the year)
Seawater zone:
Presence? (problem, no problem)
Does HAB start offshore and advect in?
Dominant species?
Duration (S, M, V, W, D, WS, WM, DW)
Frequency? (episodic, periodic, persistent, unknown)
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Mixing zone:
Presence? (problem, no problem)
Does HAB start offshore and advect in?
Dominant species?
Duration (S, M, V, W, D, WS, WM, DW)
Frequency? (episodic, periodic, persistent, unknown)
FW zone:
Presence? (problem, no problem)
Does HAB start offshore and advect in?
Dominant species?
Duration (S, M, V, W, D, WS, WM, DW)
Frequency? (episodic, periodic, persistent, unknown)
For each zone above, determine expression level (Bricker et al., 2003, p.48)
Aggregate the zone results into an estuary-wide result (see Aggregating Zone Results,
above).
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Take your scores from the above sections and aggregate as so:
Average the numerical results of the primary symptoms.
Take the highest of the numerical results of the secondary symptoms.
Final OEC: Use this decision table from Bricker et al. (2003) to determine the
overall OEC score from the above two numbers.
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III. Future Outlook
Based on foreseeable population trends, management plans, sewage treatment regulations
and technology, trends in agriculture and other changes in watershed uses, determine
expected future nutrient pressures (Bricker et al., 2003, p.50; Bricker et al, 2005, p.18):
Increase
Decrease
No change
Determine Future Outlook score using above answer and susceptibility as determined in
INFLUENCING FACTORS section. (Bricker et al., 2003, p.52)
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IV. Overall Classification Grade
Use the matrix below to determine which of five grades the estuary receives. (Bricker et
al., 2003, p.53)
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References
Bricker S.B., C.G. Clement, D. E. Pirhalla, S.P. Orlando, and D.R.G. Farrow. 1999.
National Estuarine Eutrophication Assessment. Effects of Nutrient Enrichment in the
Nation’s Estuaries. NOAA, National Ocean Service, Special Projects Office and National
Centers for Coastal Ocean Science, Silver Spring.
Bricker, S.B., J.G. Ferreira, and T. Simas. 2003. An Integrated Methodology for
Assessment of Estuarine Trophic Status. Ecological Modelling 169: 39-60.
Bricker S.B., D. Lipton, A. Mason, M. Dionne, D. Keeley, C. Krahforst, J. Latimer, J.
Pennock. 2005. Improving methods and indicators for evaluating coastal water
eutrophication: a pilot study in the Gulf of Maine. Report to NOAA-CICEET. 66p.
Smith, S.V. 2003. Preliminary NOAA estuarine typology database.
YSI Incorporated. Undated, ca. 2002. 6-Series Environmental Monitoring Systems
Manual. Yellow Springs, Ohio.
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