Campfield and Houde: Ichthyoplankton community structure and comparative trophodynamics 
15 
in the plankton. Although mean concentrations of Bos- 
rnina and calanoid copepods did not differ significantly 
in 2000 and 2001, their representation in larval diets 
did differ. For example, larval alewife and white perch 
in the freshwater region consumed higher proportions 
of Bosmina in 2000, whereas larval striped bass in the 
salt-front region consumed more Bosmina in 2001. The 
calanoid copepod Acartia occurred down-estuary of the 
salt front in the oligohaline region and was equally 
abundant during the two years but was common in diets 
of moronid larvae only in 2000. We could not explain why 
consumption of Acartia differed between the two years 
but note that moronid larvae in 2001 strongly preferred 
Eurytemora as prey. 
Feeding incidences (72-97%) in larvae and mean 
numbers of prey per gut (typically 5-50 zooplankters) 
were high in the salt front-ETM of the Patuxent River. 
In the estuarine transition zone, the two calanoid cope- 
pods Eurytemora a f finis and Acartia sp. were important 
and generally were positively selected prey. Only small 
alewife larvae, which have small mouth gape, did not 
select these relatively large calanoid copepods. The 
abundant cladoceran Bosmina also was important prey 
of fish larvae, although it usually was not positively se- 
lected. Combined contributions of the important, larger 
prey Bosmina and Eurytemora to diets of larger larvae 
were highest within and up-estuary of the salt-front- 
ETM. 
The copepod Eurytemora affinis was the most impor- 
tant food of white perch and striped bass larvae in the 
Patuxent River. It also was the most important prey 
consumed by these larvae in the salt-front-ETM of 
upper Chesapeake Bay and was particularly important 
in high freshwater-flow years (Shoji et ah, 2005; Mar- 
tino and Houde, 2010). This copepod is hypothesized to 
play a vital role in supporting the nutrition of larval 
fishes in the salt-front transition regions of Chesapeake 
Bay (North and Houde, 2006) and its tributaries, a 
role similar to that proposed for the calanoid cope- 
pod Sinocalanus sinensis and larval Japanese sea bass 
Lateolabrax japonicus in the transition region of the 
Chikugo River estuary (Islam and Tanaka, 2005; Shoji 
and Tanaka, 2007). 
Although concentrations of fish larvae are often high, 
with taxa overlaps in the salt front-ETM region, it is 
not certain that feeding competition occurs. Larval 
fish populations generally are not capable of signifi- 
cantly grazing down zooplankton prey resources (Pepin 
and Penney, 2000). Still, it is possible that foraging 
interactions are intense where larvae are highly con- 
centrated. In the estuary, diet overlaps among larvae 
may be of greatest consequence in the salt front-ETM 
region where larvae of estuarine species such as naked 
goby and anadromous species such as white perch and 
striped bass are abundant and have a common prefer- 
ence for Eurytemora as prey. Despite differences in the 
hydrographic conditions and prey available to larvae in 
the Patuxent River, the abundances of juvenile striped 
bass, white perch, and alewives in the summers of 2000 
and 2001, based on monitoring surveys conducted by 
Maryland Department of Natural Resources (http:// 
www.dnr. state. md.us/fisheries/juvindex/index.html, ac- 
cessed April 2010), were similar, indicating that envi- 
ronmental conditions in these two years were not suf- 
ficiently different to generate the high (up to 100-fold) 
recruitment variability seen in the 25-yr survey time 
series. 
Hydrographic features define assemblages and, in 
part, control trophic relationships of icththyoplankton 
and zooplankton within estuarine transition zones. The 
larvae of many fish species and abundant zooplank- 
ton prey coexist here, within and up-estuary of the 
salt front and ETM. High dietary overlap is indicative 
of strong trophic interactions among larval fish taxa. 
Despite the potential for competition, larvae in this 
transition region may have a trophodynamic advan- 
tage derived from high prey densities and the benefits 
of hydrodynamic retention. Together, these conditions 
provide a relatively stable environment that is favor- 
able for larval production and survival in the estuarine 
transition region. 
Acknowledgments 
The project was supported by Maryland Sea Grant 
(NOAA award no. NA16RG2207, Maryland Sea Grant 
R/F-90) and by National Science Foundation Ocean- 
ography Awards 0002543 and 0453905. J. Bichy, 
J. Boynton, A. Chandler, S. Gibson, E. Martino, E. 
North, B. Pearson, and J. Shoji assisted with various 
aspects of this research. We thank J. Olney (deceased) 
and D. Bilkovic at the Virginia Institute of Marine 
Sciences for assisting with larval fish identifications. 
We also thank members of the University of Maryland 
Center for Environmental Science Research Fleet for 
assistance in surveys aboard RV Pisces, RV Orion, and 
RV Aquarius. E. North and E. Martino reviewed early 
versions of the manuscript. This article is contribution 
no. 4444 of the University of Maryland Center for Envi- 
ronmental Science. 
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