- Synthesize the physical, chemical, and biological data, collected
in Chesapeake Bay during 1995-2000 as part of the Trophic Interactions
in Estuarine Systems (TIES web site) project (funded as part
of the National Science Foundation’s Land Margin Ecosystem (LMER) program).
- Provide ecological forecasts to agencies involved in Chesapeake Bay
- Extend products and approaches developed to benefit management in
a broad class of estuaries.
The comprehensive data products and forecasting tools being produced require
an interdisciplinary synthesis of physical, chemical, and biological data
collected at numerous spatial and temporal scales with a variety of technologies
(e.g., acoustics, CTDs, OPCs, ADCPs, remote sensing). During 2005, spatially-explicit
bioenergetics-based modeling was conducted to assess habitat suitability
for Chesapeake Bay fishes during spring, summer, and fall 1996-2000. One
model focused on bay anchovy, the dominant zooplanktivore in Chesapeake
Bay and a primary forage species, and the other on striped bass, the dominant
predator in this system. In both models, we used a spatially explicit approach
to explore how temperature, food availability and dissolved oxygen influenced
habitat suitability of these species, as measured by growth rate potential
(i.e., the expected growth of an individual of known size under a suite
of known habitat conditions). Overall, our modeling suggested that oxygen
availability can severely reduce habitat suitability of bay anchovy in Chesapeake
Bay during summer by limiting access to important zooplankton resources
that appear to use the low-oxygen zone as refuge. This phenomenon was particularly
evident in the deep, mid-region of the Bay, where about two-thirds of the
water column became severely hypoxic (< 2 mg/l) during summer. Habitat
suitability for striped bass also was low deep-water habitat, relative to
shallow-water habitat, in mid-portions of the Bay, owing to hypoxia in deeper
Bay Anchovy Modeling Exercise
Bay anchovy. Maps of adult bay anchovy growth rate potential (GRP;
surrogate measure of habitat suitability) have been generated for Chesapeake
Bay during Spring, Summer, and Fall 1996, 1999, and 2000 along the entire
latitudinal extent of the Bay. Although some interannual variation in
the distribution of habitat quality was evident, some striking generalizations
could be drawn:
- Spring (Figure 1, top four panels): Growth rate
potential (habitat quality) was greatest during spring across the entire
Bay, owing primarily to high zooplankton biomass that was accessible
to bay anchovy because of well-oxygenated waters (› 3 mg/l) throughout
most of the water column. In fact, ›90% of the modeled cells (cell dimensions:
~ 1 km x 1-m depth) had a positive habitat suitability value (i.e.,
a potential for positive growth; GRP › 0) during spring 1996, 1999,
- Summer (Figure 1, middle four panels): Habitat quality
was typically lowest during summer, especially in the deep, mesohaline
(mid-portion) of the Bay, owing to reduced oxygen availability (‹ 3
mg/l). This reduction in habitat quality was evident despite optimal
temperatures for bay anchovy growth. Although ample zooplankton (prey)
resources existed in the mesohaline region of the bay, they were typically
unavailable to bay anchovy; the highest levels of zooplankton prey occurred
in deep, hypoxic waters, which were not suitable for high levels of
bay anchovy consumption (dissolved oxygen levels ‹ 3 mg/l are stressful
to bay anchovy). Thus, reduced oxygen availability may indirectly limit
bay anchovy growth by reducing access to prey resources during summer.
Indeed, observed distributions of bay anchovy during summer (as measured
by hydroacoustics) closely matched the observed distribution of dissolved
oxygen (and hence, GRP), thereby supporting our model’s predictions.
Further, when the model was run without any oxygen effects on bay anchovy
foraging during summer, average bay anchovy GRPs were significantly
higher because of heightened access to zooplankton prey. Ultimately,
these results indicate that dissolved oxygen availability may severely
limit production potential of bay anchovy during summer months, which
in turn, may have ramifications for higher predators (e.g., striped
bass, bluefish). Importantly, habitat quality for bay anchovy during
summer can be variable. For example, during 1996 and 2000, the number
of cells with positive GRP was typically low during summer (‹ 51%),
whereas the clear majority of cells (88%) had a positive GRP during
1999 (Table 1). Thus, factors (e.g., nutrient
loading, temperature, wind) that influence the spatial extent of oxygen
availability also can indirectly influence habitat quality for bay anchovy
- Fall (Figure 1, bottom four panels): Habitat quality
for bay anchovy tended to be intermediate during fall months as a result
of enhanced oxygen availability in bottom waters relative to summer,
but lower zooplankton biomass relative to spring. Similar to summer
observations, inter-annual variability in GRP was evident during fall.
For example, owing to delayed fall mixing, anoxia was still prevalent
during fall 2000 sampling, which in turn, caused ~30% of the modeled
cells to have a negative growth potential (Table 1).
In summary, it appears that reduced oxygen availability can limit habitat
quality (as indexed by GRP) for bay anchovy, primarily during summer and
occasionally during fall. Most affected are deep, mesohaline waters of
the mid-Bay. Habitat quality in the shallower upper and lower reaches
of the bay, as well as the surface waters of the mid-Bay, tended to be
high (positive GRP), regardless of season. Ultimately, these results suggest
that efforts to minimize hypoxia might have a positive effect on important
zooplanktivorous prey species such as bay anchovy.
Table 1: Summary statistics of adult (age-1+) bay
anchovy growth rate potential, a measure of habitat quality, in Chesapeake
Bay during spring, summer, and fall, 1996, 1999, and 2000. The sample
size (N), mean (X), standard deviation of the mean (SDx), and median (M)
are presented for each cruise, as is the percentage of grid cells with
a positive growth rate potential for each cruise (%pos).
Figure 1. Maps of temperature, dissolved oxygen,
and zooplankton biovolume (biomass), and growth rate potential (GRP),
a measure of habitat quality, in Chesapeake Bay during spring (top four
panels), summer (middle four panels), and fall (bottom four panels) of
1996. Only results from daytime cruises are shown because 1) bay anchovy
feed primarily during daytime, and 2) zooplankton distributions during
nighttime are not reflective of actual availability, owing to diel vertical
Data Processing and Analysis Progress
- Completed processing all axial transects from Chesapeake Bay during
spring, summer, and fall 1996, 1997, and 2000
- Completed processing all lateral transects conducted during summer
and fall in these years (some spring transects also have been processed).
li›Processed all acoustics data associated with midwater trawling trawls
conducted during 1995-2000.
- Compiled all physical and lower trophic level data associated with
- Generated surface and vertical-profile maps of distributions of relative
fish biomass across the bay
- Explored multivariate statistics and spatially-explicit bioenergetics
modeling to examine how habitat availability (e.g., temperature, oxygen,
food) influences the distribution and growth rate potential of Chesapeake
- A full bioenergetics-based growth rate potential model has been developed
for bay anchovy (results presented at the annual American Fisheries
Society meeting during August 2004), as well as for striped bass.
Ludsin, S.A., X. Zhang, L.W. Florence, M.R. Roman, and S.B. Brandt. 2003.
A multi-scale analysis of factors that influence spatial distributions
of bay anchovy in Chesapeake Bay. American Fisheries Society, Quebec City, Canada.
Florence, L.W., S.A. Ludsin, W.C. Boicourt, M.R. Roman, and S.B. Brandt.
2003. Effects of the Chesapeake Bay hydraulic control point on zooplankton
and bay anchovy distributions. American Fisheries Society, Quebec City, Canada.