Road salts as environmental constraints in urban pond food webs.

Freshwater salinization is an emerging environmental filter in urban aquatic ecosystems that receive chloride road salt runoff from vast expanses of impervious surface cover. Our study was designed to evaluate the effects of chloride contamination on urban stormwater pond food webs through changes in zooplankton community composition as well as density and biomass of primary producers and consumers. From May - July 2009, we employed a 2×2×2 full-factorial design to manipulate chloride concentration (low = 177 mg L(-1) Cl(-/)high = 1067 mg L(-1) Cl(-)), gray treefrog (Hyla versicolor) tadpoles (presence/absence) and source of stormwater pond algae and zooplankton inoculum (low conductance/high conductance urban ponds) in 40, 600-L mesocosms. Road salt did serve as a constraint on zooplankton community structure, driving community divergence between the low and high chloride treatments. Phytoplankton biomass (chlorophyll [a] µg L(-1)) in the mesocosms was significantly greater for the high conductance inoculum (P<0.001) and in the high chloride treatment (P = 0.046), whereas periphyton biomass was significantly lower in the high chloride treatment (P = 0.049). Gray treefrog tadpole time to metamorphosis did not vary significantly between treatments. However, mass at metamorphosis was greater among tadpoles that experienced a faster than average time to metamorphosis and exposure to high chloride concentrations (P = 0.039). Our results indicate differential susceptibility to chloride salts among algal resources and zooplankton taxa, and further suggest that road salts can act as a significant environmental constraint on urban stormwater pond communities

Analysis of variance (ANOVA) on mean values of gray treefrog (<i>Hyla versicolor</i>) days to metamorphosis and analysis of covariance (ANCOVA) on mean values of mass at metamorphosis with days to metamorphosis as covariate.

<p>df = degrees of freedom; <i>F</i> = f-ratio; <i>P</i> = p-value; values in bold indicate statistical significance (p<0.05); values in bold indicate statistical significance (p<0.05).</p

Repeated measures analysis of variance (RMAVNOVA) with compound symmetry covariance structure on mean values of log₁₀ transformed phytoplankton abundance and periphyton abundance and analysis of variance (ANOVA) on mean values of log₁₀ transformed total zooplankton density.

<p>df = degrees of freedom; <i>F</i> = f-ratio; <i>P</i> = p-value; values in bold indicate statistical significance (p<0.05).</p

Non-metric multi-dimensional scaling (NMDS) results representing mesocosms receiving high (filled points) and low (open points) chloride.

<p>Points closer together indicate higher taxonomic similarity than those further apart. Centroids for each treatment reside under the “High” and “Low” labels. The shaded areas are the 95% confidence ellipses for the centroids. Each treatment group is bounded by the convex hull to aid in visualizing compositional spread about the centroids. The NMDS required three axes to produce a satisfactory stress (0.159). Therefore, axis 1 vs 2 (A) and 1 vs 3 (B) are portrayed. Note the scales of each are the same.</p

2×2×2 factorial design applied to 40 outdoor mesocosms (n = 5 per treatment combination) with a road salt treatment (low or high), tadpole treatment (present or absent) and inoculum treatment (collected from low or high conductance stormwater pond).

<p>2×2×2 factorial design applied to 40 outdoor mesocosms (n = 5 per treatment combination) with a road salt treatment (low or high), tadpole treatment (present or absent) and inoculum treatment (collected from low or high conductance stormwater pond).</p

Mean abiotic environmental variables measured in all mesocosms (n = 20 per chloride treatment) from study days 0–47± standard error.

<p>Mean abiotic environmental variables measured in all mesocosms (n = 20 per chloride treatment) from study days 0–47± standard error.</p