Date of Award

Fall 2020

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Wilfred Wollheim

Second Advisor

Anne Lightbody

Third Advisor

Ken Sheehan

Abstract

Low dissolved oxygen in rivers threatens fish populations, aquatic organisms, and the health of entire ecosystems. River systems with high fluvial wetland abundance and organic matter may result in high respiration that in conjunction with low re-aeration rates, lead to low surface water oxygen conditions. The increasing abundance of beaver ponds in many areas may exacerbate this phenomenon. This study aims to understand the impact of fluvial wetlands, including beaver ponds, on dissolved oxygen (D.O.) and metabolism in the Ipswich and Parker River Watersheds, MA, USA. In several fluvial wetland dominated systems, we measured diel D.O. and metabolism in the upstream inflow, the surface water transient storage zones of fluvial wetland sidepools, and at the outflow to understand how the wetlands modify dissolved oxygen. D.O. was also measured longitudinally along entire surface water flow paths (0.5 – 1.5 km long) to determine patterns of D.O. along continua with varying abundance and distribution of fluvial wetlands relative to channel-dominated systems. Nutrient samples were also collected at each scale to understand how their patterns were related to D.O.. D.O. percent saturation within the fluvial wetlands showed large diurnal variation, approaching anoxia at night and super saturation during the day. Diurnal variations also occurred in the downstream outflow, but were lagged and attenuated. The magnitude of diurnal variations was determined by flow conditions, which affect the level of inundation and interaction between advective and transient storage zones. Daily metabolic rates (GPP and ER) were higher within wetlands and at their outflows than at the channelized inflows. Wetlands of substantial size located in close proximity (< 100m apart) to one another served to decrease D.O. along the stream continuum. Large reductions in D.O. concentrations caused by wetlands also corresponded with lower nitrate levels exiting the wetlands in areas of high N input. Recent rebound in beaver populations and beaver pond abundance contributes to shorter channelized reaches between fluvial wetland environments, preventing re-aeration, resulting in maintenance of anoxic conditions along stream continua. Understanding D.O. behavior throughout river systems has important implications for the ability of river systems to remove anthropogenic nitrogen.

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