Intratidal Variability of Water Quality in the Damariscotta River, Maine

Brandon Lieberthal,Kimberly Huguenard,Lauren Ross,Zhilong Liu

doi:10.3390/w11122603

Abstract

The Damariscotta River Estuary in midcoast Maine, which houses over forty hectares of American oyster aquaculture, is characterized by several bathymetric sills, constrictions, and headlands. The geography, hydrology, and biochemistry of the estuary are closely intertwined, as its irregular shape contributes to spatially varying patterns of tidal flow, which are in turn responsible for sediment suspension. This project explores the spatial and temporal variability of the water level and current cycles of the estuary and how they are correlated to measures of water quality, such as turbidity, salinity, pH, and oxygen concentration. From July to November 2016, absolute pressure data from twelve sites were collected along the river, from which a tidal elevation time series was computed. In addition, velocity current profiles, water quality data, and wind data were obtained from surface buoys, and transects were collected in three major regions along the river. The lower and mid reaches of the estuary have a vertical shear structure of tidal flow, as high salinity water enters the estuary along the bottom during flood phase, is uplifted during the slack period, and flushed out at the surface during ebb phase. In this region, overtides are relatively weak compared to upstream, dissipation near the bottom is minimal, and turbidity and chlorophyll oscillate out of phase. North of the Glidden Ledges constriction, a headland causes the shear structure to become lateral, where dissipation is elevated during the flood phase throughout the entire water column. Dissipation has a quarter-diurnal harmonic due to increased turbulence during peak ebb and peak flood, which results in disproportionately high intratidal amplitudes in turbidity and chlorophyll. Overall, the bathymetry and tidal patterns of the estuary help to explain why the upper reaches tend to be more viable for shellfish aquaculture.

Highlights

The Damariscotta River Estuary is home to over 40 hectares of aquaculture, primarily American oyster, along with blue mussel, scallops, and seaweed [1]
Channel complexities contribute to intratidal harmonics of water elevation and current velocity, which in turn result in flood–ebb asymmetries
The research presented in this paper demonstrates that frictional forces imposed by an irregular bathymetry influence tidal harmonics with a disproportionately significant effect on the analogous harmonics of turbidity, salinity, and pH

Summary

Introduction

The Damariscotta River Estuary is home to over 40 hectares of aquaculture, primarily American oyster, along with blue mussel, scallops, and seaweed [1]. The river produces the majority of Maine’s oysters, between one million and three million units per year, and has a financial impact of millions of dollars each year [2]. The Damariscotta River features archaeological evidence of the negative impacts of climate change on shellfish. Oyster middens are mounds of human-disposed oyster shells, which line the edges of the river near Newcastle and Damariscotta and were formed from 200 BC to 1000 AD [3]. Increasing salinities and corresponding predators (oyster drill) and diseases (dermo and vibrio) were introduced to the system via climate change effects, leading to the extinction of the local oyster population, which has since rebounded. The Gulf of Maine waters are warming faster than 99%

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