labs_title

Nutrient Loading and Meteorological Conditions Explain Interannual Variability of Hypoxia in Chesapeake Bay

Y. Zhou, D. Scavia and A.M. Michalak

The Chesapeake Bay, the largest and the most productive estuary on the East Coast of the United States, experiences regular summertime hypoxic (i.e. low oxygen) conditions that have substantial ecological impacts. We present a statistical approach for mapping the 3-dimensional extent of hypoxia and throughout the year, and show that nitrogen loading, the predominance of winds along the southwest/northeast axis, and precipitation (acting as a surrogate for unmonitored nitrogen loading) explain 85% of the interannual variability in hypoxia. Results suggest that a one-third reduction in monitored nutrient loads may not be sufficient to achieve a corresponding reduction in hypoxic conditions, contrary to what had been suggested in previous studies.


Figure: Stacked bar plot of variables explaining the April to October mean hypoxic volume from 1985 to 2010. The dominant wind effect (dashed bar) is calculated as the ratio of the duration of southwesterly and northeasterly winds. The gray bars represent a constant (1.94 km3) that remains the same throughout the 26 yr period. Note that these gray bars reach to different mean hypoxic volumes in different years simply because they start at different negative values representing the dominant wind effect for each year. Red crosses and lines represent conditional-realization-derived April to October mean hypoxic volume estimates and their uncertainties, respectively. Note that data from 2007 and 2008 were not used in the analysis, and those years are therefore represented using lighter colors to distinguish them from the data used in the regression analysis.

Abstract

We use geostatistical universal kriging and conditional realizations to provide the first quantitative estimates, with robust estimates of uncertainties, of the seasonal and interannual variability in hypoxic volume in Chesapeake Bay, covering early April to late October for 1985 to 2010, and explore factors controlling that variability. Results show that the time when the hypoxic volume reaches its maximum has moved from late to early July over the examined period, but that there is no trend in the seasonal-maximum hypoxic volume itself. No significant trend was found in the timing of onset of hypoxia, but the end of the hypoxic period has moved from October to September. Including nutrient loading from the Rappahannock River in addition to the Susquehanna and Potomac Rivers is found to be beneficial for explaining the interannual variability of hypoxia. Overall, January to May total nitrogen loads from these three rivers, April to August southwesterly and northeasterly winds, and April and May precipitation explain > 85% of the seasonally averaged interannual variability in hypoxic volumes. Southwesterly winds affect hypoxia by increasing vertical stratification, while precipitation likely acts as a surrogate for nonpoint sources of nitrogen downstream from monitoring stations. The relative contribution of nutrient loading to the overall interannual variability suggests that 28–35% reductions in monitored nutrient loads may not be sufficient to achieve a corresponding reduction in hypoxic conditions as had been suggested in previous studies, at least in the short term.

Zhou, Y., D. Scavia, A.M. Michalak (2014) "Nutrient Loading and Meteorological Conditions Explain Interannual Variability of Hypoxia in Chesapeake Bay", Limnology & Oceanography, 59(2), 373-384, doi: 10.4319/lo.2014.59.2.0373.