Campfield and Houde: Ichthyoplankton community structure and comparative trophodynamics 
13 
Table 2 
Linear regressions evaluating larval total length (TL, mm) and ontogenetic state (0 L ) in relation to logarithmic mean prey length 
(PL, pm) and trophic niche breadth (S) for common larval taxa in the Patuxent River, 2000-01. r 2 = coefficient of determination. 
Model 
r 2 
P 
Alosa pseudoharengus (alewife), n=135 
Log e prey length on larval length 
PL = 
1.90 + 0.02TL 
0.17 
0.01 
Niche breadth on larval length 
S = 
0.15 + 0.01TL 
0.06 
0.12 
Log e prey length on ontogenetic state 
PL = 
1.54 + 0.01Q L 
0.16 
0.01 
Niche breadth on ontogenetic state 
S = 
0.02 + 0.01O L 
0.06 
0.14 
Morone saxatilis (striped bass), n = 165 
Log t , prey length on larval length 
PL -- 
2.58 - 0.01TL 
0.02 
0.43 
Niche breadth on larval length 
S - 
0.17 + 0.01TL 
0.07 
0.08 
Log e prey length on ontogenetic state 
PL 
= 2.73 - 0.01O L 
0.02 
0.32 
Niche breadth on ontogenetic state 
S-- 
= 0.01 + o.oio L 
0.06 
0.11 
Morone americana (white perch), n=200 
Log t . prey length on larval length 
PL 
= 1.94 + 0.04TL 
0.49 
0.01 
Niche breadth on larval length 
S 
= 0.39 - 0.01TL 
0.03 
0.27 
Log^ prey length on ontogenetic state 
PL 
1.49 + 0.01O L 
0.45 
0.01 
Niche breadth on ontogenetic state 
S 
= 0.41 -0.01O L 
0.01 
0.55 
Gobiosoma bosc (naked goby), n=133 
Log^ prey length on larval length 
PL 
= 2.07 + 0.03TL 
0.18 
0.05 
Niche breadth on larval length 
S 
= 0.13 + 0.05TL 
0.60 
0.01 
Log e prey length on ontogenetic state 
PL 
= 1.82 + 0.01O L 
0.19 
0.05 
Niche breadth on ontogenetic state 
S 
= 0.50 + 0.01O L 
0.56 
0.01 
perature was the most important variable explaining 
larval fish distributions and abundances (Harris et al., 
1999) and salinity was most important in the Tanshui 
River estuary, Taiwan (Tzeng and Wang, 1993). Larval 
fish distributions and abundances in estuaries tend 
to be most responsive to salinity-related factors (e.g., 
salt-front location), which can be strongly linked to 
precipitation and freshwater flow. In the Patuxent River 
subestuary, we found that concentrations of particular 
zooplankton prey, which also are responsive to salinity 
and temperature conditions, were significant in explain- 
ing ichthyoplankton abundances and distributions. 
In multiple regression models, the location of the salt 
front usually was a good indicator of larval concentra- 
tions for most taxa included in the models (Table 1). 
Additionally, concentrations of anadromous fish larvae 
were highest where densities of the cladoceran Bosmina 
longirostris and calanoid copepods were high. These 
zooplankters, which are important prey for fish larvae 
in Chesapeake Bay (Setzler et ah, 1981; Shoji et al., 
2005; North and Houde, 2006; Martino and Houde, 
2010), were significant in explaining concentrations of 
larvae of anadromous species in the Patuxent River, 
but not the estuarine naked goby. Goby larvae tended 
to occur where concentrations of copepod nauplii were 
high down-estuary of the salt front. 
Concentrations of zooplankton that are potential prey 
were significant explanatory variables in regression 
models for all ichthyoplankton taxa in 2000, but were 
less important in 2001. The reasons for the differences 
between years are unclear, but may be explained in 
part by the greater variability and intensity in estua- 
rine physics (river flow, salt front location) in 2001. In 
the Hudson River, Limburg et al. (1999) reported that 
cohorts of larval moronids co-occurring temporally and 
spatially with zooplankton blooms, including Bosmina, 
had higher recruitment potential. In Chesapeake Bay 
and its tributaries, temperature and freshwater flow 
may be of equal or greater importance (Rutherford and 
Houde, 1995; North and Houde 2001), although recent 
evidence has identified a strong relationship between 
recruitment success of striped bass and a temporal- 
spatial match of zooplankton production in the ETM 
region (Martino and Houde, 2010). 
Trophic interactions 
Larval diets in the estuarine transition zone were influ- 
enced by the composition of the zooplankton commu- 
nity, which differed in 2000 and 2001. On average, the 
location of the salt front was similar in both years. 
But, more variable temporal trends in river flow and 
temperature in 2001 affected the timing, intensity, and 
spatial distribution of copepod and Bosmina production 
that in turn influenced larval diets. Cyclopoid copepods, 
which were 16 times more abundant in 2000 than in 
2001, were common in larval fish diets only in 2000. 
Copepod nauplii and rotifers were relatively important 
in diets of larval moronids and naked goby in 2001 
when these small zooplankters were most abundant 
