98 
Michael A. Mallin 
in April to 260 mg/ 1 in July. The lack of taxonomic information in the 
literature regarding other coastal-plain systems makes comparisons 
difficult. 
A strong phytoplankton-zooplankton interaction was suggested by 
temporal population dynamics in Sutton Reservoir during 1985 (Fig. 2, 
3, and 4). Based on the apparent inverse relationship shown on the 
density figures, zooplankton grazing appeared to be a major factor 
regulating the phytoplankton. To test this relationship, correlation 
analyses were determined between important zooplankton and phyto- 
plankton taxa groups (Table 5). Copepods, cladocerans, and selected 
individual filter-feeding cladoceran taxa displayed significant linear 
inverse relationships with total phytoplankton, Chlorophyceae, and 
Cyanophyceae. These inverse relationships, along with the taxa of 
phyto- and zooplankton present, suggest grazing was a controlling 
factor in phytoplankton population dynamics. The chlorophyceae was 
dominated by small, naked cells or flagellates such as Chlamydomonas, 
Chlorella , and Selenastrum, which are considered easily accessible food 
for filter-feeding crustaceans and are suppressed during grazing (Porter 
1977, Vyhnalek 1983). The Chlorophyceae increased from 175 units/ml 
in April to 995 units/ml in May, concomitant with a major crustacean 
zooplankton decline (Table 2). The Cyanophyceae also showed a major 
summer increase, but grazing is probably not a major controlling factor 
for blue-greens, as members of this group are often difficult for 
zooplankton to ingest and digest (Porter 1977). Increased water 
temperatures (Tilman and Kiesling 1984, Lamberti and Resh 1985) or 
nutrient availability (Tilman et al. 1986) are more probable Cyanophyte 
controls in Sutton Reservoir. 
SUMMARY 
Sutton Reservoir is an estuarine-influenced coastal-plain system. 
Phytoplankton densities in 1985 were low to moderate and dominated 
by the Chlorophyceae and Cryptophyceae. A mean chlorophyll a value 
of 5.7 fjig / 1 suggests a mesotrophic state for this system. 
The zooplankton taxonomic composition was unusual compared 
with other North Carolina reservoirs and was probably a result of the 
estuarine influence of makeup water from the lower Cape Fear River 
and the reservoir’s geographic location. Dominant crustacean taxa were 
Diaptomus dorsalis and Eurytemora affinis of the copepods and 
Daphnia ambigua and Bosmina coregoni of the cladocerans. Rotifers 
were dominated by various members of the Brachionidae. Densities 
were high relative to other North Carolina impoundments and were 
dominated by rotifers. Zooplankton biomass was comparatively low 
and dominated by the crustaceans. 
