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Senior Theses 2004
Sara Grill (2004). Effects of the grass shrimp,
Palaemonetes vulgaris, on lower trophic levels in an
eelgrass bed community
Faculty advisor: Jeannine Lessmann
A mesocosm experiment was conducted to test the effects of crustacean mesograzers, the amphipod Gammarus mucronatus and the isopod crustacean mesograzer Erichsonella attenuata (in combination) and the omnivorous grass shrimp Palaemonetes vulgaris on epiphytic algae and total macro algal biomass and taxon composition. Mesocosms contained shoots of eelgrass, Zosteramarina, and running, filtered river water with the following treatments: (1) control (stocked only with eelgrass; not stocked with mesograzers or shrimp); (2) mesograzers only; (3) shrimp only; and (4) shrimp + mesograzers. At Day 13 of the 34-day experiment, epiphytic algae biomass was significantly decreased in the presence of mesograzers while there was a similar trend for the shrimp treatment. By Day 32, epiphyte biomass had almost quintupled in the control while epiphyte biomass was significantly reduced (by 92%) in. the grazer-only treatment. Epiphyte biomass was reduced by only 22% in the shrimp-only treatment, indicative of P. vulgaris' inability to effectively control large amounts of algae. These results suggest that the capacity of the mesograzers to effectively reduce epiphyte biomass was severely diminished in the presence of shrimp, indicative of a trophic cascade. Although total macro algal and diatom biomass did not vary by treatment, algal taxon composition did vary by treatment. Diatom biomass increased significantly in the shrimp-only treatment while a trend was found for diatom biomass to be reduced in the mesograzers-only treatment.
A 24-hour feeding rate experiment tested the relative effects of predation by Palaemonetes on the four main mesograzers found in eelgrass beds of the lower York River, VA.Palaemonetes vulgaris is capable of (significantly) reducing numbers of Gammarus mucronatus [eating both large (≥2.00mm) and small (<2.00mm) Gammarus, but significantly preferring the small], Erichsonella attenuata (eating significant amounts of only the smaller Erichsonella, with this effect carrying over and resulting in a total decrease in Erichsonella population as well), and the ampithoid crustacean mesograzer Ampithoe longimana (which were consumed in significant amounts only in to 'total' size class). Palaemonetes did not prey on the isopod crustacean mesograzer ldotea baltica.
By significantly reducing mesograzer populations and increasing diatom biomass, the shrimp have the potential to cause a trophic cascade resulting in decreased sea grass health and biomass. Omnivory complicates conclusions because the omnivores draw resources from two different trophic levels and have the potential to suppress populations on both consumer and algal producer levels, but the relative magnitudes of these two effects are not necessarily equal. At the same time, indirect effects of their predation on the consumers can lead to an increase in algal producer biomass. In conclusion, the grass shrimp Palaemonetes vulgaris is an effective predator, but it also causes significant change in algal composition in sea grass beds.