Water quality influences reproduction in female mosquitofish (Gambusia holbrooki) from eight Florida springs

Abstract

Contamination of freshwater ecosystems with nitrate is a growing global concern. Although nitrate pollution is recognized as a cause of aquatic eutrophication, few studies have examined the possible physiological impacts of nitrate exposure. In this study, we surveyed several reproductive variables of viviparous female Gambusia holbrooki (Poeciliidae) captured from eight springs in Florida. The eight springs represent a gradient of nitrate contamination (1-5 mg/L nitrate-nitrogen). We had two objectives in this study: to describe reproductive biology of female mosquitofish in the springs and to understand reproductive variation in the context of water quality, particularly the nitrate concentration. Our data show a significant negative association between nitrate and both dry weight of developing embryos and rate of reproductive activity among mature females. In addition, variation in Gambusia condition index and embryo number and dry weight was related to temperature variation, and hepatic weight was negatively related to dissolved oxygen concentration. Finally, we observed that many of the measured reproductive variables were interrelated and changeable, depending on gestational stage. Specifically, we provide evidence that maternal support of the embryo occurs at least during the first two thirds of gestation and that female fecundity is affected by an apparent tradeoff between embryo size and embryo number.

Figure 1

(A) Mean maternal hepatic weight, adjusted for body weight; (B) embryo wet weight; and (C) embryo dry weight plotted by embryonic stage [stages based on Hayne (1995)]. Graphs represent data pooled from all eight springs. Data at stage 3.5 were limited to a single female. Embryo weights represent the sum of the embryo and yolk sac. We did not obtain oocyte/embryo weight data at stages younger than 3 because those oocytes are small, variable in size, and possibly unfertilized. Error bars indicate ± 1 SE. *The collective mean dry weight of embryos between stages 4.5 and 8 (enclosed in box on graph) was significantly greater than the collective mean dry weights of embryos either younger or older (ANOVA: Scheffe’s post-hoc test, p < 0.05).

Figure 2

Percentage of nonreproductive, mature females sampled from Florida springs with varying nitrate concentrations. Fish were sampled during the reproductive season. Total samplings from each spring consisted of 30 mature females. r² = 0.57, p = 0.03.

Figure 3

Mean embryo number, adjusted for maternal body weight for females captured in Florida springs with varying temperatures. Graph shows mean ± 1 SE. r² = 0.76, p = 0.005.

Figure 4

Embryo dry weight (mg) for embryos taken from females captured in Florida springs with varying concentrations of nitrate. Graph shows mean ± 1 SE. r² = 0.56, p = 0.003.

Figure 5

Mean hepatic weight, adjusted for body weight, for females captured in Florida springs with varying dissolved oxygen concentrations. Graph shows adjusted mean ± 1 SE. r² = 0.85, p = 0.001.

Figure 6

Muscle estradiol concentrations for females from each spring. Graph shows mean ± 1 SE. Means are not statistically different (ANOVA, p = 0.15). Numbers at base of data columns indicate sample size.