Altering Hypoxic Extent and Duration

The movies showing the environmental and habitat variables (clams and background) done at year 19 when crabs were not present in the estuary can be found elsewhere. Only movies for the summer of year 39 are included here.

For each of the four maximum hypoxic extents (15, 30, 45 and 60%), three hypoxic durations were considered: Short, Long and Fixed. The environmental variables are reported with respect to these three durations. In summary, under both dynamic and fixed hypoxic patches, food limitation was not a primary factor governing crab population dynamics. Instead, as discussed in the paper, the crab populations were controlled primarily by an hypoxic patch, crab-cannibalism interaction.

Dissolved Oxygen

Increasing hypoxic duration from Short to Long to Fixed altered the dynamics of the hypoxic patches. Under Short and Long durations hypoxic patches started to form in the early spring (end of March to end of April, depending on the maximum hypoxic extent case) and dissipated by fall (mid September to mid October). In both cases, the numerous patches moved throughout the estuary. Under Short duration, the median hypoxic duration at >4 m depth ranged from ~3 to ~9 days between 15% and 60% maximum hypoxic extent, while under Long duration it ranged from ~2 to ~15 days. Under Fixed hypoxia, depths >4 m were continuously hypoxic from June to September, with the single hypoxic patch forming at the same time as for Short and Long.

Background Prey and Clam Biomass

Both the background prey biomass under Short and Long hypoxic durations and also clam biomass under Short and Long durations had a patchy distribution across the estuary. As hypoxic extent increases, the density of both prey resources decreased with most losses occurring in the deeper parts of the western end of the estuary. Under Fixed duration, both background and clam biomass showed large losses in the deeper parts of the estuary, with these losses extending further into shallower waters as hypoxic extent increased.

Clam Density

Short, Long and Fixed durations all indicated that the low clam biomass density in the deeper parts of the estuary was not the result of recruitment limitation. Instead, the highest clam densities actually occurred in the deeper parts, but experienced high mortality due to hypoxia.

Crab Biomass and Crab Density

Crab biomass under Short and Long durations was distributed in patches throughout the estuary in early April. Once hypoxia formed this biomass was “chased” around the estuary as the crabs avoided these hypoxic patches. Under Fixed, although some crab biomass was present in the deeper parts of the estuary in early April, once hypoxia formed all crab biomass occurred along the shorelines.

The crab biomass movies essentially represent the density of adult crabs while the crab density movies (Short, Long and Fixed) represent the density of all crabs in the estuary and thus are dominated by lowest instars (7 to 12). Crab density gradually decreased until 7^th instar recruitment started occurring in mid-August (39.63). The density of these recruits was greatest towards the mouth of the estuary.