Timescales of Benthic Macrofaunal Response to Diel and Episodic Low Oxygen in a Subtropical Estuary

Abstract

Hypoxia is one of the predominant water quality issues affecting estuaries and coastal ecosystems and its impact is often monitored using benthic macroinvertebrates. The M-AMBI (Multivariate AZTI Marine Biotic Index) is an index that meets the needs of small and large-scale monitoring as it is scalable. However, gaps remain as to the sensitivity of M-AMBI to hypoxia as few studies are available. Using Pensacola Bay in the northern Gulf of Mexico (USA) as a case study, we sought to evaluate the time scales over which benthic macrofauna respond to dissolved oxygen conditions from May through September 2017. Combined continuous DO monitoring and benthic sampling identified important differences in DO exposure on benthic habitat condition based on both the duration and frequency of low oxygen. We identified periods of 7 to 31 days as critical windows of exposure prior to a measurable benthic response, and that both duration and exposure to varying low oxygen conditions as well as the recovery period of oxygen to > 5 mg L^-1 are important to benthic habitat health. While the duration of exposure to DO from < 2 mg L^-1 to near anoxia remains an important factor in benthic health, benthic organisms can better tolerate periods of low oxygen when reoxygenation occurs after a short time interval. More research is needed to better quantify the relationship between oxygen stress and recovery on benthic habitats, particularly in systems where low DO exposure and recovery can vary over timescales of hours to days.

Keywords: Estuary; Gulf of Mexico; M-AMBI; benthic index; hypoxia; monitoring.

Figure 1.

Bathymetric map of Pensacola Bay and sampling stations PB05 and PB05E.

Figure 2.

Conceptual example of exposure type during a 31-day window for benthic sampling on 07/19/2017 at station PB05E. The exposure types represent: A.) Hours over the 31day exposure window DO was below 2 mg L⁻¹, B.) Hours over the 31day exposure window DO was above 2 mg L⁻¹, C.) Maximum DO value observed over the 31 day exposure window, D.) Mean DO averaged over the 31 day exposure window, E.) Minimum DO value observed over the 31 day exposure window. Note that these example exposure types are applied to the different exposure windows described in Table 1.

Figure 3.

Daily boxplots of DO (black lines) from continuous monitoring in 2017 at stations PB05 (top) and PB05E (bottom). M-AMBI numerical index score (red squares) and discrete DO from CTD casts (yellow circles) are also depicted at the date of sampling. Failure of the WQM at each station in September limited assessment of the fall recovery period, indicated by CTD.

Figure 4.

NMDS of benthic community abundance data for PB05 (N=10) and PB05E (N=4). Points represent site scores colored by station (top) and M-AMBI benthic habitat condition (bottom). Vectors represent species (top) and DO treatments (bottom) significantly to the ordination via permuted correlations (α = 0.05), while asterisk denotes species where p=0.06.