Hypoxia in the Baltic Sea during the last century – feedback on nutrient

LOCAL: sala de reunioes do IO,

DATA:  13 de março de 2013

HORA: 10:00h

Jacob Carstensen

Dept. of Bioscience, Aarhus University, Frederiksborgvej 399, DK-4000 

Roskilde; jac@dmu.dk jac@dmu.dk>

One of the most profound effects on the health of the Baltic Sea is the increase of hypoxia over the last century due to anthropogenic nutrient loading. Bottom water oxygen concentrations are strongly influenced by physical factors, especially the inflow of saltier, denser water. These inflows are governed by large-scale and local meteorological forcing, and have large variations in frequency and magnitude over time-scales of decades. Salt water inflows bring new supplies of oxygen to bottom waters, but at the same time enhance stratification creating larger bottom areas that experience hypoxia. However, it is the increased flux of organic material to the bottom water and sediments due to nutrient enrichment, which has disrupted the balance between oxygen supply through physical processes and oxygen consumption from decomposition of organic material. Therefore, it is believed that the extent of hypoxia has increased with discharges of nutrients from land and atmosphere over the last century, although there are only few studies quantitatively supporting this. Oxygen monitoring data from the beginning of the 1900’s suggest that hypoxia was confined to the very deep parts of the Baltic Sea, but the scarcity and heterogeneity of the sampled data (in both the

vertical and horizontal space) as well as inability to measure hydrogen sulphide complicate the assessment of hypoxic area and volume before ca. 1970, when more frequent monitoring was initiated. Since 1970 the extent of hypoxia has fluctuated in response to variations in inflows but there has been no overall trend in the Baltic Proper. Thus, the large change in hypoxia occurred before monitoring efforts really began, and confident estimates in the rate of change are difficult to achieve. We have re-analyzed the historical monitoring data by building statistical models that address the problems associated with the historical monitoring data to deliver estimates for the change in oxygen conditions, unbiased by monitoring efforts and detection limits. Our results show that in the beginning of the 20^th century hypoxia was confined to the deepest parts of the Baltic Sea with an extent less than 10,000 km^2 . From the 1930’s to the 1980’s oxygen conditions worsened with most of the water column below the halocline being affected by hypoxia, resulting in more than 60,000 km^2 dead bottoms at present.

We also show that enhanced nutrient inputs from land and atmosphere is the primary reason for this trend, which is modulated by oscillations in saltwater inflows at decadal scale. The expansion of hypoxia has strong consequences for the nutrient cycling, sustaining a vicious cycle with enhanced nutrient releases from the sediments that fuel algal blooms and subsequently maintain hypoxic conditions.

Fonte: Secretaria do IO