Ecological thresholds of hypoxia and sedimentary H2S in coastal soft-bottom habitats: A macroinvertebrate-based assessment

Abstract

Ecological thresholds of dissolved oxygen (DO) and sedimentary hydrogen sulfide (H2S) for macrozoobenthos were examined during a 30-month monitoring of two stations in a highly eutrophic canal in inner Tokyo Bay, Japan. Bottom DO and H2S concentrations fluctuated seasonally, and were significantly correlated with water and sediment temperatures. Red tide-derived phytodetritus was a major source of sediment organic matters in the canal bottom, and the sediment became highly reduced and sulfidic condition in warmer months (sedimentary H2S; up to 8.5 mM). Dominant opportunistic taxa, including polychaetes and amphipods, were eliminated under low DO and high H2S conditions (i.e., population thresholds), and devastation of community structure occurred at 2.4–3.3 mg l−1 DO and 1.8–2.7 mM H2S (i.e., community thresholds). To maintain ecosystem function in anthropogenically degraded habitats and ensure colonization by macrozoobenthos throughout the year, DO and H2S levels should be maintained below these thresholds.