The impact of hypoxia on bioturbation rates in the louisiana continental shelf, northern Gulf of Mexico

Abstract

Variation in the spatial and temporal extent of hypoxia in coastal bottom waters of the northern Gulf of Mexico leads to changes in benthic community structure and sediment physical properties. Past and present benthic community structure determines what types of biogenic structures are present in the sediment as well as faunal mixing rates. Therefore, hypoxia has an important effect upon bioturbation. This study focuses upon the effects of hypoxia on bioturbation specifically on the continental shelf of Louisiana, where hypoxia has become an important issue due to its seasonal reoccurrence and increasing expansion across the northern Gulf of Mexico over the past 30 years. In this project, characteristics of biogenic structures in the sediment including number, diameter, and depth are correlated with benthic communities dwelling in hypoxic, intermittently hypoxic, and normoxic conditions using non-destructive Computed Tomography (CT) imagery of sediment cores and Sediment Profile Imaging (SPI) photography. Biogenic structures are also correlated with sediment physical properties, bioturbation rates, and bioturbation behaviors (dilator or compactor) of benthic invertebrate fauna. Initial data for this project was collected during two cruises along the continental shelf of Louisiana, the first in April 2009 and the second in September 2009. Four different were chosen for sampling using bottom water oxygen concentration data from the Louisiana Universities Marine Consortium (LUMCON) and archived sediment type data. These four sampling sites represent normoxic and hypoxic provinces with a consistent sediment type. The provinces consist of a normoxic site (NO) that has experienced hypoxia less than 25% of the time and briefly hypoxic (BH), frequently hypoxic (FH) and hypoxic (HO) sites that have experienced hypoxia greater than 25% of the time. We expect areas of rapid deposition (organics and inorganic sediment) and low bottom water oxygen to be colonized by a few organisms that are concentrated near the sediment-water interface, remaining above the depth of the redox potential discontinuity (RPD). In contrast, well oxygenated areas are expected to have developed diverse benthic communities that have attained the equilibrium stage of succession, which includes larger, deeper burrowing fauna. The diverse, deeper-burrowing benthos helps to create and maintain a deeper RPD. As a consequence of the vertical zonation and faunal diversity differences between stressed and recovered areas, several sediment properties may be affected. SPI photographs show that RPD depths in the four provinces vary from 2 to 6 cm below the sediment surface in the spring, before hypoxia has developed. The x-radiographs depict the extent of recent sedimentation and bioturbation at each province. Featured in the x-radiographs are: event layers, shell fragments, voids above and below event layers, vertical, horizontal, and complex burrows. Preliminary radiochemistry results for the April 2009 cruise indicate shallow depths of sediment mixing at all sites on a temporal scale of ~1 year (Be-7), though provinces HO and FH show a deeper mixing depth (2.25 cm) over this period than provinces BH or NO (1.25 and 1.75 cm, respectively). This may reflect a competitive advantage of discrete macrobenthic organisms who are better able to tolerate hypoxic conditions, a rapid re-colonization of previously hypoxic bottom sediments, or both. Results from the April 2009 cruise are presented as baseline data to establish benthic macrofauna densities and sediment properties before development of hypoxia. A second cruise was conducted in September to assess the effects of seasonal hypoxia at these same four provinces.