Sub-lethal effects of hypoxia on harpacticoid copepod reproduction

Areas of hypoxia are found in coastal areas worldwide, and have become increasingly widespread. These areas vary in their duration and dissolved oxygen concentration from occasional diurnal hypoxia, as found in Corpus Christi Bay, Texas, seasonal hypoxia as in the northern Gulf of Mexico, to continuous hypoxia as found in oceanic oxygen minimum zones. The effects of exposure to low dissolved oxygen (DO) depend on the duration of exposure, the DO concentration and an organism’s tolerance to hypoxic conditions. Most studies have focused on lethal effects of hypoxia by comparing the abundance of benthic organisms and the species composition of benthic communities between hypoxic and normoxic areas. Sub-lethal effects of such as changes in reproduction may occur at less severe hypoxic conditions (by definition), but may still have effects at the population level. The goal of this study is to examine the sub-lethal reproductive effects of low DO on harpacticoid copepods. The life-history traits and reproductive biology of meiobenthic harpacticoid copepods make this group of organisms useful as test organisms to measure the sub-lethal effects of hypoxia on reproduction. It is hypothesized that changes in reproductive traits may be observed at DO concentrations higher than those that cause lethal effects because of the high energetic cost of female harpacticoid reproduction which may result in reduced fitness.
Laboratory studies were conducted to examine the effects of low DO concentrations on survival, egg production, and the number and size of eggs. Harpacticoid population abundance, biomass and the abundance of ovigerous females were measured from field samples collected across sites that varied in their degree and duration of exposure to low DO. To contrast the effects of hypoxia in environments with different hypoxic exposures, field studies were carried out in the northwestern Gulf of Mexico in an area with near constant summer hypoxia, and in southeast corner of Corpus Christi Bay, Texas which experiences intermittent hypoxia often for less than one hour in the early morning hours during summer. At both field sites total copepod abundance, biomass and the abundance of ovigerous females were reduced at the most hypoxic site type compared the reference sites. In Corpus Christi Bay, total and ovigerous female abundance varied with exposure to hypoxia, but the relative declines in abundance and biomass were less dramatic than in the northern Gulf of Mexico. In the northern Gulf of Mexico, the percentage of ovigerous females within the total harpacticoids collected was reduced (p<0.001) to 3% at the Intermediate and 1% at the Hypoxic sites during the summers, but in the fall of 2007 at all transects had similar percentages (p = 0.81) of ovigerous females which ranged from 13 to 16%. Reference sites had a higher number of families with ovigerous females, and the relative abundances of those families were more evenly distributed at Reference sites compared to Hypoxic and Intermediate transects. Patterns of ovigerous female abundance with respect to DO concentration were similar across numerous harpacticoid families found at the study sites.
Harpacticoid copepods in laboratory studies tolerated near anoxic DO conditions for longer than 120 hours. Average survival rates of the harpacticoids Schizopera knabeni and Nitokra affinis during periods of low DO with elevated ammonium concentrations were less than 50%, while survival rates for the other treatments were near 100%. The formation of an egg mass was reduced in the near anoxia treatment, but egg clutches did not form at all in the treatment that combined near anoxia and 10 micromolar ammonium.
The DO concentration and presence of ammonium were important factors to the severity of population decline that were illustrated by population estimate calculations. Population estimates based on laboratory measurements of Nitokra affinis resulted in calculations of reduced survival and reproductive rates related to hypoxic exposure greatly decrease the potential population of the group, which may affect the copepod population’s ability to recover from hypoxic events. The effects of hypoxia on harpacticoid copepods are the result of the length of exposure, the concentration of low dissolved oxygen and exposure to other chemical fluxes that increase during periods of hypoxia. The measurement of reproductive indicators, such as the presence of an egg mass and the proportion of ovigerous females in a population, could be used to indicate the adverse environmental effects of low dissolved oxygen exposure. The studies from this dissertation are the first to document reproductive effects of low dissolved oxygen on harpacticoid populations in the field. Laboratory studies within this dissertation indicate the interaction between ammonium and hypoxia on survival and reproduction in harpacticoids. Future studies are needed to further determine the effects of hypoxia on the whole-life cycle of harpacticoid copepods.