Influence of Resource Pulses Following Stochastic Disturbances on Aquatic Food Webs

Resource pulses, defined as brief, low frequency events of large magnitude increases in resource availability, are found in a variety of ecosystems including islands, forests, deserts, and streams and are becoming a prevailing theme in ecology. Resource pulses can substantially contribute toward ecosystem productivity by supplying ecosystems with nutrients that support additional primary and secondary production, which can subsequently affect higher trophic levels. In the aquatic realm, resource pulses of anadromous Pacific salmon Oncorhynchus spp. have garnered most of the attention from ecologists due to their ability to deliver inland aquatic and terrestrial environments an important marine-derived nutrient pulse. However, the contribution of pulsed resource events in other aquatic ecosystems has remained vastly unexplored. Common carp Cyprinus carpio is a highly abundant invasive fish in shallow eutrophic lakes throughout much of the world and is associated with deleterious effects on aquatic ecosystems due in part to nutrient subsidies provided through bioturbation and excretion. Shallow lakes dominated by common carp are often subject to winter hypoxia, periodically resulting in stochastic mass mortality events. These natural disturbances may result in an important resource pulse through carcass decomposition, providing increased nutrient availability and primary productivity, enhancing eutrophication processes, and stabilizing the turbid-water state. However, how resource pulses derived from stochastic disturbances affect eutrophic food webs is unknown. The objectives of this study were: 1) to estimate the range of common carp carrion biomass and nutrients released following winterkill and their resulting effects on nutrient availability and primary and secondary productivity; 2) to evaluate how ecosystem effects resulting from resource pulses following stochastic disturbances compare to continuous subsidies supplied by live common carp; 3) to compare how recipient ecosystem trophic state influences response magnitude of producers and consumers to resource pulses; and 4) to determine how resource pulse magnitude influences ecosystem response size. For my first objective, I evaluated common carp carrion biomass, nutrient availability, primary productivity, and primary consumer production in six natural eutrophic lakes following a natural stochastic disturbance. Lakes that were presumed to experience a complete winterkill had large nutrient pulses with an estimated 264 kg/ha of common carp carrion that released 1,187 g/ha total phosphorus and 6,648 g/ha total Kjeldahl nitrogen whereas lakes presumed to only experience partial fish kills had a moderate pulse of 23 kg/ha of common carp carrion that released 104 g/ha total phosphorus and 585 g/ha total Kjeldahl nitrogen. Lakes experiencing pulses had higher nutrient concentrations, turbidity, phytoplankton, zooplankton, and benthic invertebrate production and lower periphyton production compared to undisturbed control lakes with effects typically greater for large compared to moderate pulses. Differences in nutrient availability and productivity among systems were detected shortly after ice-out and persisted throughout the summer. Nutrient pulses stabilized pelagic production (phytoplankton and total zooplankton) but did not affect temporal stability of other parameters. Stable isotopes indicated that energy flow shifted and consumers became more reliant upon pelagic production in systems receiving pulses, likely due to increased pelagic and decreased benthic primary production in these systems. Common carp bioturbation subsidies and carrion pulses may both have substantial but different effects on food webs and community structure but controlled experiments comparing ecosystem effects of continuous versus pulsed pathways are lacking. For my second objective, I examined how common carp nutrient subsidies from bioturbation and excretion and pulses from decomposition differed in their ability to affect and sustain ecosystem productivity and alter stability and energy flow. I quantified changes in water chemistry, biological production, ecosystem stability, and energy pathways in mesocosms over 56 days among four common carp treatments [continuous subsidy (live carp), pulse (dead carp), disrupted (live carp removal), and a non-carp control]. Nutrient availability and primary production were highest in pulsed, intermediate in continuous, and lowest for disrupted and non-carp control systems. Continuous, and to a lesser extent pulsed, systems had decreased water clarity and sago pondweed Potamogeton pectinatus coverage. Increased nutrient availability and primary production from pulses supported higher densities of large-bodied zooplankton and macroinvertebrates. Biotic and abiotic variables following common carp removal quickly returned to control levels and water clarity substantially improved. Water clarity, phytoplankton, and Daphnia spp. Were more temporally variable in pulsed systems whereas temporal variability of other measured variables was generally similar among treatments. Biotic material collected from pulsed and subsidized systems were enriched in δ15N suggesting that common carp derived nutrients were supporting increased productivity whereas δ13C were depleted, suggesting a switch to more pelagic energy pathways, likely due to extensive phytoplankton production in these systems.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation - University Access Only

Discipline thesis:degree_discipline

Natural Resource Management

Year

2011

Author dc:creator

• Weber, Michael John

Contributors dc:contributor

• Michael L. Brown

• Natural Resources and Conservation

Statement dc:rights

• Copyright © 2011 Michael John Weber. All rights reserved.

Language dc:language

en

Repository record dc:identifier

https://openprairie.sdstate.edu/etd/598

OAI identifier oai:identifier

oai:openprairie.sdstate.edu:etd-1598