Estuarine oxygen dynamics: What can we learn about hypoxia from long‐time records in the Delaware Estuary?

Abstract

Hypoxia and anoxia occurred in the upper Delaware Estuary throughout much of the 20th century and diminished over the past several decades. I reviewed 30 yr of data from my laboratory's research efforts, 40 yr of consistent monitoring data from a multistate agency, results from inconsistent data collection from the past century, and anecdotal information to construct a long-time picture of the decline and increase of dissolved oxygen concentrations (DO) in the urban region of the estuary. The primary cause of the DO decline appeared to be inputs or allochthonous materials from urban sources (reduced nitrogen and carbon). In spite of extremely high nutrient concentrations, excess algal production did not influence DO anywhere along the tidal freshwater stretch or the saline portion of the well-mixed Delaware Estuary; and it does not have an influence today. The nutrient loading to the Delaware Estuary is very high, yet the typical signs of eutrophication are not obvious. Based on a model of apparent oxygen utilization, the Delaware Bay apparently had higher primary production 50 yr ago, a time when nutrient concentrations were as high or higher than today, shellfish and finfish production were apparently also higher, and DO was close to saturation. This analysis is offered as guidance in assessing and managing estuarine eutrophication, which is too often considered narrowly to be the result of inadvertent overfertilization by nutrients or a single nutrient element.

Season‐specific trends and linkages of nitrogen and oxygen cycles in Chesapeake Bay

“…Such limits on NH 4 + accumulation for a given nitrogen input may be associated with oxygenation and associated nitrification. Tight linkages between NH 4 + availability and nitrification rate have been observed in many other systems worldwide (Soetaert et al 2006;Sharp 2010 concentrations have only increased in sub-pycnocline waters during late-summer also suggests that internal production (i.e., nitrification) is the key mechanism driving the increase, as opposed to new watershed inputs that primarily impact surface waters during winter-spring. The late-summer (August to September) period is a time when hypoxic and anoxic conditions are usually diminished, as stratification weakens with declining water temperature and elevated wind speeds during the late summer and fall (Goodrich et al 1987;Wilson et al 2008) and vertical mixing of oxygen increases replenishment of bottom waters.…”

Long‐term variations in the dissolved oxygen budget of an urbanized tidal river: The upper Delaware Estuary

“…Yoshiyama and Sharp [2006] showed that primary production could be modeled well as a function of chlorophyll, light, and temperature, accounting for about 80% of the variability in the upper estuary; nutrients did not show any predictive power. In a further examination, Sharp [2010] found little change in primary production from the early 1980s to the early 2000s in the upper estuary. Water column respiration measurements are far fewer in the estuary.…”

“…Primary production is highly seasonal, with summertime rates averaging about 3 mol O 2 m −2 month −1 and wintertime rates essentially zero. Differences between the two time periods are modest, as suggested by Sharp [2010]; though it appears that primary production is a little lower in the more recent period, statistically significant differences were not found when there were five or more data points in a given month. Since J O 2 = P − C, where P is net primary production and C is nonalgal oxygen consumption, C is simply P − J O 2 .…”

“…Biological oxygen loss processes in the upper Delaware Estuary include heterotrophic respiration, autotrophic respiration, and nitrification. By combining our estimates of net oxygen consumption with the extensive set of primary production measurements described by Pennock and Sharp [1986], Yoshiyama and Sharp [2006], and Sharp [2010], it is possible to estimate the nonalgal component of oxygen consumption (heterotrophic respiration plus nitrification). The primary productivity measurements were made by measuring radiocarbon uptake during 24 h incubations.…”

“…The cleanup of the Delaware River was based on a minimum allowable 24 h average oxygen concentration of 3.5-6.0 mg L −1 (109-187 mmol m −3 ), or 42-72% of saturation at 25 ∘ C), a standard that is still in place today Delaware River Basin Commission [2013]. Increasing dissolved oxygen concentrations have been recorded from the 1940s up until the 1990s, but from the 1990s until the present summer dissolved oxygen levels remained between 50 and 100 mmol m −3 below saturation [Albert, 1988;Sharp, 2010]. The reason for the lack of continual dissolved oxygen concentration increases is an area of concern, with occasional negative impacts from low dissolved oxygen levels still occurring, including fish kills [NJDEP, 2010].…”