Geology topics

Melanie Hayn

Publications and source records attributed to Melanie Hayn.

3 recordsLinked to original sources

USGS2013

Metabolism of a nitrogen-enriched coastal marine lagoon during the summertime

We measured metabolism rates in a shallow, nitrogen-enriched coastal marine ecosystem on Cape Cod (MA, USA) during seven summers using an open-water diel oxygen method. We compared two basins, one directly receiving most of the nitrogen (N) load (“Snug Harbor”) and another further removed from the N load and better flushed (“Outer Harbor”). Both dissolved oxygen and pH varied greatly over the day, increasing in daylight and decreasing at night. The more N-enriched basin frequently went hypoxic during the night, and the pH in both basins was low (compared to standard seawater) when the oxygen levels were low, due to elevated carbon dioxide. Day-to-day variation in gross primary production (GPP) was high and linked in part to variation in light. Whole-ecosystem respiration tended to track this short-term variation in GPP, suggesting that respiration by the primary producers often dominated whole-system respiration. GPP was higher in the more N-loaded Snug Harbor. Seagrasses covered over 60 % of the area of the better-flushed, Outer Harbor throughout our study and were the major contributors to GPP there. Seagrasses covered 20 % of the area in Snug Harbor for the first 5 years of our study, and their contribution to GPP was relatively small. The seagrasses in Snug Harbor died off completely in the 6th year, but GPP remained high then and in the subsequent year. Overall, rates of phytoplankton GPP were relatively low, suggesting that benthic micro- and macro-algae may be the dominant primary producers in Snug Harbor in most years. Net ecosystem production in both Snug Harbor and the Outer Harbor was variable from year to year, showing net heterotrophy in some years and net autotrophy in others, with a trend towards increasing autotrophy over the 7 years reported here.

Massachusetts↗

USGS2013

Exchange of nitrogen and phosphorus between a shallow lagoon and coastal waters

West Falmouth Harbor, a shallow lagoon on Cape Cod, has experienced a threefold increase in nitrogen load since the mid- to late 1990s due to input from a groundwater plume contaminated by a municipal wastewater treatment plant. We measured the exchange of nitrogen and phosphorus between the harbor and the coastal waters of Buzzards Bay over several years when the harbor was experiencing this elevated nitrogen load. During summer months, the harbor not only retained the entire watershed nitrogen load but also had a net import of nitrogen from Buzzards Bay. During the spring and fall, the harbor had a net export of nitrogen to Buzzards Bay. We did not measure the export in winter, but assuming the winter net export was less than 112 % of the load, the harbor exported less than half of the watershed nitrogen load on an annual basis. For phosphorus, the harbor had a net import from coastal waters in the spring and summer months and a net export in the fall. Despite the large increase in nitrogen load to the harbor, the summertime import of phosphorus from Buzzards Bay was sufficient to maintain nitrogen limitation of primary productivity during the summer. Our findings illustrate that shallow systems dominated by benthic producers have the potential to retain large terrestrial nitrogen loads when there is sufficient supply of phosphorus from exchange with coastal waters.

Buzzards Bay, West Falmouth Harbor↗

USGS2012

Tidal and groundwater fluxes to a shallow, microtidal estuary: Constraining inputs through field observations and hydrodynamic modeling

Increased nutrient loading to estuaries has led to eutrophication, degraded water quality, and ecological transformations. Quantifying nutrient loads in systems with significant groundwater input can be difficult due to the challenge of measuring groundwater fluxes. We quantified tidal and freshwater fluxes over an 8-week period at the entrance of West Falmouth Harbor, Massachusetts, a eutrophic, groundwater-fed estuary. Fluxes were estimated from velocity and salinity measurements and a total exchange flow (TEF) methodology. Intermittent cross-sectional measurements of velocity and salinity were used to convert point measurements to cross-sectionally averaged values over the entire deployment (index relationships). The estimated mean freshwater flux (0.19 m 3 /s) for the 8-week period was mainly due to groundwater input (0.21 m 3 /s) with contributions from precipitation to the estuary surface (0.026 m 3 /s) and removal by evaporation (0.048 m 3 /s). Spring–neap variations in freshwater export that appeared in shorter-term averages were mostly artifacts of the index relationships. Hydrodynamic modeling with steady groundwater input demonstrated that while the TEF methodology resolves the freshwater flux signal, calibration of the index–salinity relationships during spring tide conditions only was responsible for most of the spring–neap signal. The mean freshwater flux over the entire period estimated from the combination of the index-velocity, index–salinity, and TEF calculations were consistent with the model, suggesting that this methodology is a reliable way of estimating freshwater fluxes in the estuary over timescales greater than the spring–neap cycle. Combining this type of field campaign with hydrodynamic modeling provides guidance for estimating both magnitude of groundwater input and estuarine storage of freshwater and sets the stage for robust estimation of the nutrient load in groundwater.

Massachusetts↗