An Investigation of the Effect of Turbidity on the Diel Vertical Migration of Zooplankton in the Chincoteague Bay, VA.

James Bergenti, Department of Biology, York College of Pennsylvania

Introduction• Sedimentation in marine ecosystems plays a role in water

clarity. Most species of fish rely heavily on vision for predation. Therefore, increased turbidity can reduce predation efficiency by obscuring prey detection (Cezilly 1992).

• Vertical excursions on a diel cycle are exhibited in most species of pelagic dwelling zooplankton. This is commonly known as diel vertical migration (DVM). This vertical movement is primarily dictated by light (Haney 1993) and generally occurs at dawn and dusk.

• A central hypothesis explaining diel vertical migration is the predator avoidance hypothesis. This hypothesis suggests that zooplankton migrate into deeper water during the day to avoid being eaten by visual predators searching for food in shallow water, while moving to shallower, food rich waters at night (Bollens and Frost 1989).

• The degree to which these vertical movements are performed may be influenced by turbidity. Increased turbidity may allow species of zooplankton to persist at certain depths and reduce the need of vertical migration (Roman 2001).

• Chincoteague Bay, located in between Assateague Island and the eastern shore, is the largest coastal lagoon on the eastern shore of Maryland, and has an average depth of less than 3m. Zooplankton located here can be collected from the surface during daylight, a trend not consistent with typical migration behavior. The purpose of this study was to investigate the relationship between turbidity and migration behavior in zooplankton from this estuary.

HypothesesH0: There will be no difference in diel vertical migration behavior with varying levels of turbidity.

H0: There will be no difference in diel vertical migration behavior between zooplankton samples collected at varying depths in the water column.

http://www.madrimasd.org/blogs/ciencia_marina/2009/11/30/129464

Samples collected via a horizontal or vertical

zooplankton tow

3 replicates for each turbidity level: Low (~1NTU), Medium (~25NTU), High (~40 NTU)

12hr. day/night cycle for approximately 48 hours in lab

to acclimate

Day and night samples collected from top, middle, and bottom

of cylinders

Preserved with 2 mL of buffered formalin

Identified and counted under a microscope

Methods Results

Fig. 7. Percentage of total surface sample population found at the top of the water column in low, medium, and high turbidities. A non-parametric and Kruskal-Wallis analysis were run. There was no significant difference among the populations between each turbidity level. (P= 0.3012) The error bars represent the standard error of the mean

Fig. 8. Percentage of zooplankton found at the top depth in low turbidity from surface and water column samples. A nonparametric Mann-Whitney test was run on the data. There was no significant difference between the two populations. (P= 0.7) The error bars represent the standard error of the mean.

High

Medium

Low

Surface

High

Medium

Low

Water Column

Top

Middle

Bottom

Discussion• Although some movement was observed, there was

no significant difference in the percentage of zooplankton found at the top between turbidities or between surface and water column copepods.

• Factors that may have influenced the results:

Fig. 1. A-C: Percentage of total surface population found at each depth between day and night samples in low, medium, and high turbidities. D-F: Percentage of total water column population found at each depth between day and night samples at low, medium, and high turbidities. V represents the strength of diel vertical migration behavior. The error bars represent a 95% confidence interval.

Future Research• Sampling at various times of the year may produce a

more diverse sample of zooplankton.

• This experiment was isolated in a lab. Performing this experiment in the field may produce different results, as the zooplankton would be exposed to natural conditions.

• It is known that this migratory behavior of zooplankton is heavily influenced by the type of predator. Examining the relationship of DVM and various predators may help in the understanding of the mechanisms behind this behavior.

Literature Cited1. Bollens, S. M., Frost, B.W., (1989) Zooplanktivorous Fish and Variable Diel Vertical Migration in the Marine Planktonic Copepod Calanus pacificus. Limnology and Oceanography, Vol. 34, No. 6, pp. 1072-10932. Cezilly, F. 1992. Turbidity as an ecological solution to reduce the impact of fish-eating colonial waterbirds on fish farms. Colonial Waterbirds 15:249–252.3. Haney, J.F. 1993, Environmental control of diel vertical migration behavior. Limnology and Oceanography, 39: 1-174. Roman, M.R., Holliday, D.V., Stanford, L.P. 2001, Temporal and spatial patterns of zooplankton in the Chesapeake Bay turbidity maximum. Mar Ecol Prog Ser, 213: 215-227

A

B

C

D

E

F

• The samples were dominated by a single species: Acartia tonsa.

• The dominant predators in the Chincoteague Bay could be nonvisual feeders. Oysters and mussels, for example, are common in the bay.

• The amount of fish, and thus the risk of predation, may vary between seasons. Bollens and Frost (1989) showed that the extent of DVM is directly related to the amount of the dominant species of zooplanktivorous fish at the time.

• The Chincoteague Bay is a highly productive estuary. Food may be available throughout the entire water column, not just at the top or bottom.

http://earthobservatory.nasa.gov/Features/OceanProductivity/printall.php

V= 0.01

V= 0.06

V= 0.05

V= 0.1

V= 0.16

AcknowledgementsI would like to thank Dr. Nolan for the time and expert advice she dedicated to

this study.

http://sites.wff.nasa.gov/code250/story31.html

http://ian.umces.edu/imagelibrary/displayimage-6163.html

http://landsat.gsfc.nasa.gov/images/archive/f0015.html

V= 0.07