Dissolved oxygen dynamics in salt marsh pools and its potential impacts on fish assemblages

In order to determine the dynamics of dissolved oxygen (DO) in salt marsh pools and its effect on their respective fish assemblages, DO was measured in 5 high-salinity pools in a Spartind alterniflora marsh in southern New Jersey over ten 24 h periods, from 2 July to 22 October 1990, to determine daily and monthly fluctuations. Typically, DO peaked at approximately 16:00 h, and reached a minimum by about 05:00 h. The greatest variation occurred in mid-July, when DO ranged from over 20 ppm at midday, to 0 ppm in the early morning. From mid- to late August, DO remained at very low concentrations (<1.0 ppm) for long periods (11 7 ± 1.7 h). DO concentrations varied among marsh pools, but there was no interaction between pool and time of day. The young-of-the-year (YOY) of the dominant 5 species of fishes in these pools were tested for tolerance and behavioral responses to low DO in laboratory aquaria. Fundulus heteroclitus survived the longest at low DO, and Mendidia menidia was least tolerant. Aquatic surface respiration (ASR) was the primary behavioral response to low DO for all species tested. All species, including Cyprinodon variegatus, Lucania parva, and M. beryllina, survived at DO < 0.5 ppm when they used ASR; however, when fish were denied access to the air-water interface, only F. heteroclitus was able to survive more than 1 h. All species initiated ASR at ca. 1 ppm, with the exception of M. menidia, which began ASR at 2.6 ppm and survived only at higher concentrations of DO. In summary, although the salt marsh pools showed extreme diurnal variability in DO, with rapid drops from supersaturation to anoxic conditions, all of the fish species, with the exception of M. menidia, were well adapted to survive these fluctuations. This lack of tolerance to low DO by M. menidia may seasonally limit its occurrence in marsh pools but this is not the case for the congener M. beryllina or the other resident fishes.