Impacts of White Perch Introductions on Trophic Dynamics: Paleolimnological Record of Zooplankton Grazing and Nutrient Cycling

Student Investigator
Kristin Ditzler, Graduate Student
School of Biology and Ecology, UMaine

Project Synopsis
Impacts from fish introduction on size selective predation and alteration of nutrient cycling will be examined using paleolimnological techniques.  This analysis will provide insight into the long-term developments, past conditions, and biological response to fish introduction at the whole lake scale.  These results will be compared to a whole-lake biomanipulation project on East Pond; a shallow, mesotrophic lake located in the Belgrade Chain of Lakes in central Maine.  The goal of this study is to gain a better understanding of the complex mechanisms by which fish introductions affect plankton dynamics and to assess the applicability of biomanipulation as a management approach for improving water quality.

Introduction
Fish introduction to lake ecosystems produces long-lived changes in zooplankton and algal communities via cascading interactions in food webs (Leavitt, 1989).  Trophic cascades occur when impacts from the upper levels of the food web cause corresponding changes in lower levels.  Cascading influences of fish typically result from strong predation by planktivorous fish on zooplankton, resulting in reduced grazing pressure on algae (Brooks and Dodson, 1965).  In some cases, these cascading effects can lead to substantial increases in algal abundance, or algal blooms, which cause aesthetic and water quality problems for lake users as well as degrading ecosystem health (Jeppesen, 1990).  The mechanisms underlying these trophic cascades are complex and many factors can alter the response of food webs to manipulation.  For example, besides direct trophic interactions, introduced fish can affect lower trophic levels through alterations in the cycling of nitrogen and phosphorus (Vanni, 1997).  As a result, untangling the interactions between fish introductions and water quality is difficult.

This type of cascading influence of an introduced fish is thought to have contributed to the reduced water quality in East Pond, Maine.  Between approximately 1920 and 1950, white perch, a planktivorous fish, was introduced to East Pond, a shallow, mesotrophic lake located in the Belgrade Chain of Lakes in central Maine.  Since this time there has been an increase in the frequency of noxious algal blooms and a decrease in water clarity.  To address this, the Maine Department of Environmental Protection has implemented a lake biomanipulation project in which white perch are being removed from the lake.  The removal of an abundant planktivorous fish from the food web can reduce predation by fish on the zooplankton community, which is predicted to favor large bodied zooplankton that have an increased grazing efficiency on algae.  An increase in algal consumption can reduce biomass and reduce the potential for noxious algal blooms.  This is known as a top-down control on water quality dynamics. 

However, preliminary results from the biomanipulation on East Pond suggest responses by the plankton community that are counter to those expected from changes to top-down predation effects.  Direct influences on lake water quality such as nutrients (total phosphorus) and zooplankton grazing pressure (cladoceran biomass) were monitored in East and North Ponds prior to fish removal in 2004 to 2006, and after fish removal in 2007 and 2008. In the first year of white perch removal, cladoceran body size declined (counter to predictions) and returned to pre-removal sizes in the second year. Algal blooms were less severe in the two years of perch removal, but not lower than all years monitored prior to fish removal.  Interpretation of these results is difficult due to limitations in the amount of data used to quantify pre and post conditions as well as the lack of replication due to the scale of the experiment.

Paleolimnological techniques, using lake sediment records to study long term changes to the lake ecosystem, can be used to analyze the long-term, complex interactions that occur in lakes due to alterations in fish community.  For example, an analysis of the sediment record of East Pond revealed that after 1950 (approximate timing of perch introduction) the zooplankton community showed an increase in body size with an increase in the size divergence (counter to predicted top-down predation effects).   Additional paleolimnological studies can provide information about fish effects on trophic dynamics in respect to long-term developments, past conditions, and biological response at the whole lake scale. 

I propose examining the historical patterns of algal and zooplankton communities, as recorded in lake sediments, to test for long-term effects of white perch introduction to Maine lakes. In lakes with known dates of white perch introduction and others without perch introductions, sedimentary records will be analyzed for changes in fossil pigments (indicator for shifts in the algal community) and zooplankton (indicator for shifts in cladoceran size structure and species composition).  In addition to community changes, possible shifts in water clarity and in nutrient availability will also be examined.  Thus, the objective of this study is to analyze the long-term impacts of planktivorous fish introduction on Maine lakes.

Objectives

• Use paleolimnology to assess impacts of white perch introduction on:
• Zooplankton community structure
• Algal community composition
• Nutrient limitation and changes to nutrient cycling (inferred from the ratio of carbon, nitrogen, and phosphorus in bulk sedimentary organic material)
• Compare these results to the East Pond biomanipulation project and assess the applicability of biomanipulation as a treatment for improving water quality.

Methods
Sediment cores will be taken from 3 lakes with known dates of white perch introduction and two lakes of similar morphometry, watershed characteristics, and nutrient concentrations, but no white perch introduction. The coring sites will be determined in consultation with the Maine Department of Environmental Protection at a meeting on March 30.

Sediments will be collected by boat with a gravity corer. The top 20 cm of each core will be sectioned into 0.5 cm increments in the field. This length of core typically captures the last 100-300 years in Maine lakes.

The cores will be dated using 210Pb dating in the Physics Department at UMaine. Fossil pigments will be analyzed with high-performance liquid chromatography. Cladoceran ephippia size will be used as an indicator of zooplankton body size and will be measured using imaging software in conjunction with microscopic examination. The elemental (carbon, nitrogen, and phosphorus) composition of bulk sedimentary organic material will be determined by combustion analysis (for carbon and nitrogen) or digestion and colorimetric analysis (for phosphorus).

Impact of project
Analyzing ecosystem communities before and after food web manipulation on the scale of hundreds of years will provide the larger perspective needed in understanding fish effects on trophic dynamics.  The comparison of records from multiple lakes will provide insight into fish introduction across broad spatial scales.  Management of water quality requires a better understanding of the large-scale and long-term impacts of planktivorous fish introduction.  Results from this study will guide management decisions aimed at treating water quality through food web manipulation.

Expected deliverables

• An annual report to the Mitchell Center.
• Publication of results in a journal such as Freshwater Biology.
• Presentation at the 2010 Maine Water Conference, as well as the 2010 Graduate Student Expo at UMaine and the 2010 joint meeting of ASLO & NABS.

Investigator qualifications
I graduated from James Madison University with a Bachelors of Science degree in Biology in the Spring of 2006.  During my time at James Madison, I worked with geographic information systems to analyze airborne mercury contamination.  I also studied the effects of riparian strips in agricultural areas, using macroinvertebrate diversity and density as assessment criteria.  After graduation, I began working with the Susquehanna River Basin Commission as the Biologist for the Watershed Restoration and Protection division.  During this time I worked on a number of projects ranging from stormwater management to acid mine drainage remediation. I began working on the East Pond project during June of 2008.  I collected and processed water quality data along with phytoplankton and zooplankton samples during the open water season of 2008.  I am studying under Jasmine Saros to gain experience in processing and analyzing lake sediment cores.