Temporal responses of coastal hypoxia to nutrient loading and physical controls

Abstract

Abstract. The incidence and intensity of hypoxic waters in coastal aquatic ecosystems has been expanding in recent decades coincident with eutrophication of the coastal zone. Worldwide, there is strong interest in reducing the size and duration of hypoxia in coastal waters, because hypoxia causes negative effects for many organisms and ecosystem processes. Although strategies to reduce hypoxia by decreasing nutrient loading are predicated on the assumption that this action would reverse eutrophication, recent analyses of historical data from European and North American coastal systems suggest little evidence for simple linear response trajectories. We review published parallel time-series data on hypoxia and loading rates for inorganic nutrients and labile organic matter to analyze trajectories of oxygen (O 2 ) response to nutrient loading. We also assess existing knowledge of physical and ecological factors regulating O 2 in coastal marine waters to facilitate analysis of hypoxia responses to reductions in nutrient (and/or organic matter) inputs. Of the 24 systems identified where concurrent time series of loading and O 2 were available, half displayed relatively clear and direct recoveries following remediation. We explored in detail 5 well-studied systems that have exhibited complex, non-linear responses to variations in loading, including apparent "regime shifts". A summary of these analyses suggests that O 2 conditions improved rapidly and linearly in systems where remediation focused on organic inputs from sewage treatment plants, which were the primary drivers of hypoxia. In larger more open systems where diffuse nutrient loads are more important in fueling O 2 depletion and where Correspondence to: W. M. Kemp (kemp@umces.edu) climatic influences are pronounced, responses to remediation tended to follow non-linear trends that may include hysteresis and time-lags. Improved understanding of hypoxia remediation requires that future studies use comparative approaches and consider multiple regulating factors. These analyses should consider: (1) the dominant temporal scales of the hypoxia, (2) the relative contributions of inorganic and organic nutrients, (3) the influence of shifts in climatic and oceanographic processes, and (4) the roles of feedback interactions whereby O 2 -sensitive biogeochemistry, trophic interactions, and habitat conditions influence the nutrient and algal dynamics that regulate O 2 levels.

Natural and human-induced hypoxia and consequences for coastal areas: synthesis and future development

“…Mesoscale intrusions of subsurface suboxic waters from the Rim Current of the Black Sea general circulation system also episodically modulate the near-bottom oxygen structure of northwestern shelf (Black Sea Commission, 2008). Longterm observations show that coastal hypoxia off large river mouths and in estuaries/bays has an important seasonal (Rabalais et al, 2001) and/or inter-annual component (Gilbert et al, 2005;Kemp et al, 2009) that can be related to climate variability as well. For example, the annual discharge of the Mississippi River is predicted to increase by 20% if the concentration of atmospheric CO 2 doubles, and nutrient load would increase as well, which will induce stronger stratification and more serious eutrophication, followed by an ever intensified hypoxia on the Louisiana Continental Shelf (Rabalais and ).…”

“…Comparative studies also clarify how biogeochemical dynamics defines the magnitude and consequences of coastal hypoxia. High-resolution measurements have enabled the establishment of cross-boundary and mechanistic understanding of the emergent, evolving and declining phases of coastal hypoxia (Kemp et al, 2009). The episodic, periodic (i.e.…”

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

“…Inflow of nutrients, especially nitrogen (N), stimulates primary production in the estuary, occasionally leading to a rise in frequency and intensity of harmful algal blooms (Weise et al 2002;Bricker et al 2008) which induce massive mortality of marine fish, birds and mammals . Organic carbon exported from the watershed and plankton produced within the estuary eventually sediments into deep estuarine waters, where enhanced microbial activity induces hypoxia and anoxia (Kemp et al 2009). Median rates of oxygen decline are more severe in a 30 km band near the coast than in the open ocean ([100 km from the coast) (Gilbert et al 2010) which also exerts a range of negative effects on marine coastal ecosystems (Diaz and Rosenberg 2008).…”

Acidification of Lower St. Lawrence Estuary Bottom Waters

“…Hypoxia occurs naturally in many coastal environments with restricted circulation, such as fjords and inland seas, but hypoxia in locations with less restricted circulation both offshore (e.g., California Current, Benguela Current) and near-shore (Diaz and Rosenberg, 2008;Gilbert et al, 2010) is increasingly being reported. This may reflect changes in ocean circulation (Gilbert et al, 2005) or increased eutrophication (Kemp et al, 2009). Shallow or seasonally stratified water columns can be ventilated by vertical mixing events, which may prevent hypoxia from becoming persistent (Rabalais et al, 2001).…”