Non-linear Responses of a Coastal Aquatic Ecosystem to Large Decreases in Nutrient and Organic Loadings

Abstract

Between 1991 and 2000, Boston Harbor, a bayestuary in the northeast USA, experienced a decrease in loadings of total nitrogen (TN), total phosphorus (TP), and particulate organic carbon (PC) of between ∼80% and ∼90%. The average concentrations of TN and TP in the harbor water column were decreased in linear proportion to the loadings. The changes to the chlorophyll-a (chl-a), PC, and bottom water DO concentrations were curvilinear relative to the loadings, with larger changes at low than high loadings. For TN and TP, the starts of the decreases in concentrations coincided with the starts of the decreases in loadings. For the three variables that showed curvilinear responses, the starts of the changes lagged by 2 to 3 years the starts of the decreases in TN loadings. Total suspended solid concentrations and water clarity in the harbor were unchanged. The study shows that for systems such as Boston Harbor, decreases in nutrient loadings will have quite different effects depending on the base loadings to the system.

Response of benthic metabolism and nutrient cycling to reductions in wastewater loading to Boston Harbor, USA

“…To describe site differences, and to facilitate cross-reference to other studies Taylor, 2010;Taylor et al, 2011), data for each site were averaged according to time periods corresponding to treatment changes (Fig. 2), These periods roughly match those previously defined by Taylor (2010) (Table 1), but exclude Period I because we have limited data from that period.…”

“…There is a great deal of interest in evaluating how successful these attempts have been and in learning how to predict the trajectory of eutrophication reversal or "oligotrophication" (Duarte, 2009;Nixon, 2009). Studies to date Taylor et al, 2011) have shown that not all estuaries respond similarly to decreases in nutrient loading, and that some systems do not revert to their original conditions. Sediments play an important role in organic matter decomposition and nutrient cycling (Nixon, 1981;Boynton and Kemp, 1985) and may impact the rate at which estuaries respond to changes in nutrient loading.…”

“…Although both northern and southern sections of the harbor have suffered from long-term wastewater inputs, water and sediment quality in some areas were more severely affected than in others, depending on proximity to these sources and to local tidal flushing (Kelly, 1997). It is important to note that despite very high nutrient loading, Boston Harbor did not experience some symptoms of hyper-eutrophication, such as nuisance algal blooms and hypoxia (Taylor et al, 2011) common to other nutrient-impacted estuaries because it is relatively shallow and has a short average water residence time (Stolzenbach and Adams, 1998). This allowed investigation of benthic responses to large changes in organic matter and nutrient loading separate from the usual accompanying conditions of bottom water oxygen stress, which tend to exacerbate the effects of high carbon loading in sediments .…”

Recovery of Danish Coastal Ecosystems After Reductions in Nutrient Loading: A Holistic Ecosystem Approach

“…However, while the eutrophication process leading to ecosystem degradation is well studied and understood, our knowledge on coastal ecosystem recovery following significant nutrient reductions is limited and suffers from scarcity of longterm large-scale ecosystem studies. Existing studies indicate that complete recovery of coastal ecosystems to a previous baseline is rarely a realistic assumption (Lotze et al 2011;Duarte et al 2015) because degradation and recovery typically follow different non-linear pressure-response pathways and the interaction of multiple pressures may lead to shifts in baseline conditions (Duarte et al 2009;Carstensen et al 2011;Taylor et al 2011). …”

Modeling ecosystem processes with variable freshwater inflow to the Caloosahatchee River Estuary, southwest Florida. I. Model development

“…Anthropogenic manipulation of freshwater inflow alters salinity distributions that affect estuarine organisms and the structure of food webs (Livingston et al, 1997;Powell et al, 2003;Tolley et al, 2005;Gillson, 2011;Petes et al, 2012). Nutrient loading is a function of freshwater discharge and the incoming concentrations of N and P (Boynton et al, 2008;Greening et al, 2011;Taylor et al, 2011). Symptoms of estuarine eutrophication are often ascribed to the increased loading of dissolved nutrients (e.g.…”