Date of Award

Fall 12-16-2016

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)


Ecology and Environmental Sciences


Hamish S. Greig

Second Committee Member

Brian J. Olsen

Third Committee Member

Jasmine E. Saros

Additional Committee Members

Scott A. Wissinger


Tidal freshwaters are unique in their placement in the landscape, forming where freshwater riverine flows are sufficient to overwhelm the saline water of the incoming tide, but not strong enough to suppress tidal water-height fluctuations. Tidal freshwaters and their wetlands have been overlooked by freshwater and marine researchers alike; neither discipline considers tidal freshwaters to fall under their purview. Invertebrate communities in tidal freshwaters are thought to be species depauperate; the consensus is that they support fewer taxa than nearby non-tidal freshwaters, but little is known about how these communities are structured. This study investigated how tidal hydrology, an atypical suite of environmental conditions for a freshwater habitat, influences benthic invertebrate community and trophic structure. A comparison was made between tidal freshwater wetlands, at three tidal heights (low, mid, high), and nearby non-tidal freshwater wetlands that varied in their hydrology (temporary and permanent).

Tidal freshwater wetlands generally had lower invertebrate richness and abundance than non-tidal freshwater wetlands. However, tidal freshwater wetlands contributed a large proportion (~25%) of unique taxa to the local species pool suggesting that these wetlands may be important for supporting regional biodiversity. Communities were more strongly differentiated between wetlands of different tidal height than between hydrologically isolated inland wetlands.

Both invertebrate community structure and trophic structure exhibited zonation across the tidal height gradient. Low tidal height wetlands were typified by high abundances of a few dominant taxa, which were smaller-bodied on average than taxa found at higher tidal heights. High tidal height wetlands supported communities with more-evenly distributed abundances and supported more large-bodied invertebrates. Overall, invertebrate biomasses (body size x abundance) were greatest at wetlands of intermediate tidal height (mid) as were the biomasses of detritivorous and predatory invertebrates.

The tidal hydrology of tidal freshwater wetlands controls a suite of abiotic and biological drivers of community structure. Desiccation risk varies with tidal height due to variable inundation times. Likewise, the risk of predation (by fishes) appears to be linked to the time available for foraging during high tide. The reduction in diversity, invertebrate body size, and biomass at low tidal heights may be driven by fish predation, as these patterns are commonly observed in non- tidal freshwaters. Fish may have less access to higher tidal heights, which means larger invertebrate mesopredators could be released from predation pressure. Invertebrate biomasses at the highest tidal heights may be limited by a combination of limited foraging time (short inundation periods), high temperatures and desiccation risk, and by top-down control by terrestrial predators that utilize the marshes at low tide. Further work is needed to elucidate the relative importance and interactive effects of the abiotic and biological drivers that a likely responsible for the distinct zones of community structure and trophic structure observed across the tidal height gradient in this study.

Tidal freshwaters exhibit strong environmental gradients in a condensed geographic area and are amenable to manipulation. The broad range of abiotic and biological conditions found in tidal freshwaters may make them ideal study systems for answering broader questions in community ecology.