Our Students and Their Research

The effects of simulated predation on plankton community structure were investigated in mesocosms of three volumes. lt was hypothesized that simulated predation would modify the abundances, dynamics and structure of the plankton community and that the timing and level of these responses would be related to the levels of predation and the sizes of the mesocosms. Ambient Patuxent River water and a proportional amount of natural estuarine sediment were added to tanks having volumes of 60.3, 442 and 675-liters. Three tanks of each volume were designated for the experiment. Predation levels of 0, 10% and 30% were imposed on each volume designation. Counts of  copepods/copepodites, copepod nauplii, polychaete larvae and other zooplanktors were taken daily. A measure of fluorescence was taken every other day. Pre-predation (days 2-5) copepod/copepodite densities were significantly higher in the 675-liter and lowest in the 60.3-liter tanks. Early predation (days 6-12) copepod/copepodite densities were significantly affected by predation level but not by tank size. Copepod/copepodite abundances scaled inversely with predation level. Late predation (days 13-18) copepod/copepodite densities were significantly affected by tank volumb and predation level. Higher densities occurred in the smaller tanks under the lowest predation level. Copepod nauplii densities showed no indication of a tank size effect during the pre-predation period. Tank size did affect nauplii density during early predation, with abundances highest in the smaller tanks. During late predation both tank size and predation level had significant effects, with higher abundances in the smaller tanks and at high predation levels. There was no indication of a tank size effect on fluorescence levels during pre-predation. Tank size, but not predation level, did significantly affect fluorescence during early and late predation, with highest levels in the smaller tanks.

Submerged aquatic vegetation (SAV) is a vital part of the natural ecological functioning of estuarine environments. SAV within the Chesapeake Bay has been declining steadily, and it has had an impact on many levels within the estuarine system. Research has been and is continuing to be done to determine the causes for their decline. The Multiscale Experimental Ecosystem Research Center has been conducting studies at Horn Point Environmental Laboratory dealing with various aspects of SAV within the Bay. One portion of their research deals with the relationship of complexity to ecosystem functioning and more specifically deals with the role of grazers and predators within the SAV community. The grazers selected were an amphipod, Gammarus rnucronatus and a snail, Hydrobia sp. The predator species was a banded killifish, Fundulus diaphanus. Methods were developed to handle and quantify amphipods for means of experimental documentation and prepare the way for a correlation to be made between grazer density and epiphytic covering. 

The marine phytoplankton, Prymnesiophyceae, contains membrane Iipids, long chain C₃₇ methyl alkenones. The prymnesiophyte, Emiliania huxleyi, is an open ocean species that has shown that the degree of unsaturation within the alkenone long chain changes with temperature. The unsaturation ratio, U(K/37), shows a linear response to temperature change and is presently used as a biomarker for past and present sea surface temperatures (SST) from cores of the ocean floor. A coastal prymnesiophyte, Isochrysis galbana, also contains these alkenones. Definitive-work with I. galbana and its relation to E. huxleyi has yet to be done. I. galbana and its relation to temperature and salinity of growth was the focus of this research. I. galbana was cultured in varying temperatures at a constant salinity and also in varying salinities at constant temperature.  Alkenones were extracted and analyzed using a HPGC with FID. The relation of the U(K/73) to temperature (10-25°C) was linear and best fitted using the equation U(K/37) =  0.035T-0.352 with an r²of 0.9198. This equation is very similar to that of E. huxleyi:. U(K/37) = 0.033T + 0.043 showing a high correlation between slopes. This indicates that I. galbana may be used to predict SST along the coastlines. Results of I. galbana grown in varying salinities (10-30 ppt) show little change of U^K and help support the use of I. galbana as a biomarker for SST.

The abundance and wide, euryhaline distribution of bay anchovy Anchoa mitchilli, make it difficult to monitor migration patterns throughout its complete life history. Otolith microchemical analysis was applied to larval bay anchovy from Chesapeake Bay with the objectives to (1) develop procedures, (2) determine if otoliths of larvae have Sr/Ca ratios that are directly related to the salinity level in their region of collection, and (3) determine if changes in Sr/Ca ration have occurred in the otoliths which are indicative of larval transport or migration across salinity zones during the first several weeks posthatch. Otolith Sr/Ca ratios were analyzed for 25 bay anchovy larvae collected from five areas throughout Chesapeake Bay in June and July 1993, representing upper, middle, and lower regions of the Bay. Microchemical analysis of otoliths was carried out with a wavelength dispersive electron microprobe. There was a weak, but significant, regression of otolith Sr.Ca ratios on salinity in larval bay anchovy. It was possible to distinguish most otoliths from larvae collected in low or high salinities (i.e. 5 or 25 ppt). Variation in the Sr/Ca ratio among juvenile and larval bay anchovy otolith data showed significant overlap in Sr/Ca levels at salinities below 15 ppt and a nearly 3 fold higher Sr uptake rate for juveniles than for larvae, suggesting a possible ontogenetic effect. Laboratory-rearing experiments using known salinities, as well as the need to analyze larger sample sizes from a wider range of salinities, are necessary to confirm whether otolith microchemical analysis is applicable to understanding migration or transport of larval bay anchovy.

The occurrence of bacterial clumping in marine systems was investigated as related to the presence of flagellate predators. The factors of movement, bacterial concentration, and flagellate species were included in this investigation. The flagellate Bodo parvulis was found to cause clumping in a strain of bacteria isolated from Canary Creek in Lewes, Delaware. The role of Bodo was explored in the clumping process with specific regard to exopolymer secretions and the trophic dynamics possibly affected by them.

An investigation of ingestion rates of the estuarine and calanoid copepod A. tonsa in mesocosms of differing scales was completed. The gut florescence method was used to determine levels of gut fullness and this value was used in the empirical formula of Dam and Peterson (1988) to determine ingestion rates. Results suggest that scale does not affect rates of ingestion in A. tonsa, however, the effect of scale may be apparent in lower trophic levels, which is reflected in the trophic behavior of A. tonsa.