Vernal Pools IV: Effects of Depth and Temperature on Vernal Pool Morphospecies Abundance and Diversity Ian Hazelhoff and Erica Teasley (Biology 210—Dr. Deborah McGrath)

IntroductionVernal pools, in broad terms, are rain-fed wetlands that develop in shallow depressions over impervious surfaces (Keeley and Zelder 1998) and are a hot spot for invertebrate, salamander, and bird diversity (Scheffers, et al. 2005). Without established fish populations, vernal pools host a prolific range of “avoider” and “tolerator” species, many of which remain vulnerable to even the slightest variations in water temperature and depth regardless of their specializations (Colburn 2004). Because of the pools’ relative isolation, only these highly specialized organisms are capable of inhabiting them. The Southeastern United States, known for its pronounced granite outcroppings, has unique vernal pools that differ hydrolytically and temporally from other regions in the U.S. (Keely 1991). In particular, the period between late winter and late spring contains the highest levels biotic and abiotic change throughout the year (Colburn 2004). One notable consequence of being primarily rain-fed is that most vernal pools are nutrient poor. Because of the high surface to volume ratio of the mostly shallow vernal pools, most experience high daily variations in temperature (Keely 1991). It is unknown how localized variance in temperature and precipitation will affect the already delicate balance of vernal pool life histories in Sewanee, Tennessee. For this study, several crustacean species were examined for their direct life history relations to documented trends in vernal pool abiotic change. Copepods are small crustaceans that inhabit nearly every freshwater habitat in the United States. Their life history can range from between one week and one year, depending on the species. Huntley and Lopez (1992) showed that temperature alone explains over 90% of the variance in growth rates between the 33 species of copepod. Copepods are not the only contributor to vernal pool community structures, but are good indicators of overall ecosystem health. Uye (1981) showed that maximum copepod longevity decreases with increasing temperature, likely due to higher metabolic demands at higher temperatures. To observe these communities, we chose to take measurements of pelagic crustacean diversity and abundance. Our study sites spanned six well-documented vernal pools on the Cumberland Plateau in Sewanee, Tennessee. We hypothesized that vernal pool depth and temperature would affect pelagic morphospecies abundance and diversity over time.

Discussion

Since copepods were the most common morphospecies, their step-wise decline from winter to late spring says something about the types of copepods found in Sewanee. While some copepods are found through the winter months and into early spring, others peak just before the summer months (Colburn 2004). Crustacean morphospecies found in this study appeared to peak in abundance during the first two weeks of the study (Figure 7). This suggests that the types of specialized Copepods in Cumberland Plateau vernal pools may have adapted to cold-weather climates for optimal reproductive success. With additional data in this field, a more accurate conclusion of Copepod life history in the area could be made. Our results showed that temperature and depth have a small negative relationship (Figure 1; Correlation =-0.22). As the temperature increases depth decreased due to an increase of evaporation. However, in previous studies it has been shown that changes in depth have had a more significant correlation with precipitation than evapotranspiration (Brooks 2004). This study suggests that the effects of precipitation are a limiting factor in vernal pool species composition and abundance. When depth increased due to precipitation, an increase in water level allowed organisms to spread out, resulting in lower abundance and higher diversity per unit volume of water. Diversity and temperature have a strong negative relationship (R² = 0.9642). Based on this conclusion, an increase in water temperature would reduce the overall abundance of vernal pools. While this certainly does not hold true for all vernal pools, it serves as a data point for those on the Cumberland Plateau. Abundance and temperature have a weak positive relationship (Figure 2; R² = 0.0864); this may be due to a few species that can survive warmer water temperatures and are seen longer into spring/early summer. However, overall frequency of crustaceans decreased over time regardless of temperature change. This may be related to the differences in diapause timing among the unique populations of Sewanee. If global temperatures increase the expected 2°C to 4.5°C (Meehl et al. 2007), our results indicate that morphospecies diversity of Sewanee’s vernal pools will be negatively affected. The expected increase in heavy rains punctuated by long droughts (Meehl et al. 2007) could result in the loss of drought-resistant life stages due to runoff followed by an explosion of new, fast-developing organisms.

MethodsSite DescriptionsMushroom Pond: A medium to large sized forested ephemeral pond located approximately 300 yards off of a gravel road. Water depth in some areas can get up to 2 feet, with medium levels of pelagic clarity Breakfield Road: A large, better known and permanent ephemeral pond area.. High levels of shore brush are common at this site, with higher levels of depth. Piney Point: Located deeper into the forest and farther from possible human interactions. In addition, this site has much shallower waters, with two sections of the pond being almost separated by a dry area.  Airport Road 1, 2 and 3 Located just a short distance from several heavily traveled and paved roads. There were notably more trees in the body of the ponds than in other locations, and the water is more spread out over a larger location. Experimental At each pool:• Pelagic samples were taken at the center of the pool at three random locations,

scooping one tub full of water.• At least one representative individual was captured in a 50mL Falcon tube for each

morphospecies found, and abundance numbers were recorded for each tub.• Sample tubes were refrigerated until microscope identification could be completed.  Statistical AnalysisSpecies diversity and abundance were compared to temperature and depth using a one-way ANOVA. To compare temperature between ponds over time and depth in ponds over time, we used a two-way ANOVA with replication.

ResultsTemperature and depth have a weak negative correlation (Figure 5; Correlation=-0.22). Depth and precipitation have a very weak positive correlation (Figure 5; Correlation= 0.045). There was not a significant relationship in abundance between ponds (ANOVA; P = 0.36). However, there was a significant relationship in abundance at all ponds over time (Figure 6; ANOVA; P= 0.08). There was not a significant difference in diversity between ponds (ANOVA; P= 0.30) or at all ponds over time (ANOVA; P= 0.17). There was a significant difference in temperature between ponds over time (ANOVA; P=1.39E-07). There was not a significant difference in depth between ponds over time (ANOVA; P=0.71). There was an overall decrease in crustacean frequency over time in the measured vernal pools.

Literature CitedColburn, E.A. 2004. Vernal pools: natural history and conservation. McDonald & Woodward Publishing Company, Blacksburg, Virginia.

Connell, J.H. The influence of Interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology 42:710-23

Huntley, M.E. and Lopez, M.D.G., 1992. Temperature-dependent production of marine copepods: a global synthesis. Am. Nat. 140, pp. 201–242

Keeley, J.E. and P.H. Zedler. 1998. Characterization and global distribution of vernal pools, p. Pages 1-14 in: C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren Jr., and R. Ornduff (Editors). Ecology, Conservation, and Management of Vernal Pool Ecosystems – Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, CA. Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao, 2007: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, N Y, USA.

Scheffers, B.R., J.B.C. Harris, and D.G. Haskell. 2006. Avifauna associated with ephemeral ponds on the Cumberland Plateau, Tennessee. J. Field Ornithol. 77(2):178-183.

Figure 5: Mean temperature between vernal pools (n=20; ANOVA; p=5.69E-08) and mean depth between vernal pools (n=20; ANOVA; p= 0.71). Airport 1, Airport 2, Airport 3, Breakfield Road, Mushroom, and Piney Point vernal pools. Sewanee, TN. February and March 2011.

Figure 6: Mean abundance (n=12; ANOVA; p = 0.08) over time compared to mean water temperature over time (n= 20; ANOVA; p=5.87E-14) with standard error. Airport 1, Airport 2, Airport 3, Breakfield Road, Mushroom, and Piney Point vernal pools. Sewanee, TN. February and March 2011.

Figure 1: Gravid copepod Figure 2: Amphipod

Figure 3: Non-gravid copepod Figure 4: Aquatic spider

Figure 3. Frequency of copepods, amphipods, and rotifera (n=3) of pelagic communities in relation to time at Airport Road, Brakefield Road, Mushroom Pond, and Piney Point vernal pools, Sewanee, TN. February and March 2011.

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