Cladoceran community dynamics reflect temperature gradients in a cooling water reservoir.
Abstract: Cladoceran community dynamics in limnetic systems often are correlated with temperature patterns. We sought to relate cladoceran temporal patterns and community composition to spatial and temporal temperature patterns in a cooling water reservoir (Newton Lake, Jasper County, Illinois). Effluent released into Newton Lake creates a temperature gradient where portions of the reservoir experience summer maxima in excess of 35 C, while other portions resemble a typical temperate system. We measured temperature and collected zooplankton at four locations arranged at increasing distances from the power plant cooling water outfalls. Cladoceran community density was higher near the warm effluent during winter, although no statistically significant differences amongst sites were observed during summer. Bray-Curtis dissimilarity values and nonmetric multi-dimensional scaling suggest cladoceran communities in the reservoir respond to this altered temperature with respect to abundance and seasonality of species. Daphnia lumholtzi, an exotic cladoceran, also is present in Newton Lake and sometimes is the dominant cladoceran taxon. High abundance of D. lumholtzi seems to be unrelated to warm effluents as it occurs at all sampling locations. Although morphological features of D. lumholtzi are thought to deter depredation, temporal and spatial distance likely occurs between this species and gape-limited vertebrate planktivores, thereby reducing potential negative impacts of this species in Newton Lake.
Article Type: Report
Subject: Cladocera (Environmental aspects)
Reservoirs (Environmental aspects)
Reservoirs (Thermal properties)
Authors: Metzke, B.A.
Pederson, C.L.
Pub Date: 04/01/2012
Publication: Name: The American Midland Naturalist Publisher: University of Notre Dame, Department of Biological Sciences Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Earth sciences Copyright: COPYRIGHT 2012 University of Notre Dame, Department of Biological Sciences ISSN: 0003-0031
Issue: Date: April, 2012 Source Volume: 167 Source Issue: 2
Topic: Event Code: 310 Science & research
Product: Product Code: 4941210 Reservoirs NAICS Code: 22131 Water Supply and Irrigation Systems SIC Code: 4941 Water supply
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 287956855
Full Text: INTRODUCTION

Temperature regime is an important determinant of cladoceran (Branchipoda: Order: Cladocera) community structure and abundance (Sarma et al., 2005; Stich et al., 2005), as it is with most zooplankton taxa inhabiting limnetic systems (Mitchell and Lampert, 2000). Temperature has been linked with reproduction and mortality rates, population size structure, and cyclomorphology of zooplankton, as well as phytoplanktonic prey availability (Bush et al., 1974; U.S.E.P.A, 1974; Moran, 1981; Laws, 1993; Achenbach and Lampert, 1997; Yurista, 2000; Wetzel, 2001). Seasonality of cladocerans often is dictated by temperature regime, and typically, cladoceran populations in temperate systems experience a spring or early summer population maximum (Pennak, 1953; Threlkeld, 1979; Waite, 1981; Gerten and Adrian, 2002) and a reduction in density as temperature increases in mid and late summer (Havens et al., 2000; Laugaste and Haberman, 2005). However, this pattern can vary annually depending upon local climate variation (Laugaste and Haberman, 2005) or can be completely disrupted by permanent alterations in temperature regime (Waite, 1981), as in the case of anthropogenic thermal disturbances.

Cooling water reservoirs can create elongated periods of warm temperatures (Brigham, 1981; Gilliland, 1983) and, although system dependant, temperature change above ambient levels can be drastic and may exceed 10 C (Marcy, 1971). Although this alteration in temperature regime can be beneficial at times to aquatic organisms (Bennett and Gibbons, 1973; Dahlberg and Conyers, 1981; Tranquilli et al., 1981; Waite, 1981; Laws, 1993) it also can be detrimental to biota (Benda and Proffitt, 1973; Patrick, 1973; Bush et al., 1974; U.S.E.P.A., 1974; Laws, 1993) and, at the very least, has potential to impact seasonality of cladoceran communities by altering rates or timing of reproduction, growth, and mortality (Pennak, 1953).

Within this context we attempted to investigate the relationship of thermal discharges into Newton Lake (Jasper County, Illinois) to limnetic cladoceran community dynamics. Specifically, we hypothesize thermal effluents will create distinct cladoceran community patterns between the two arms of the reservoir, with the arm receiving thermal discharges showing increased density and taxa richness in winter and decreased density and taxa richness in summer. Of additional interest in this study is the temporal and spatial variability in Daphnia lumholtzi (Sars) populations, which are known to exist within this reservoir. An exotic zooplankter native to Asia, Australia, and Africa, D. lumholtzi is more tolerant of warm temperatures (Havens et al., 2000; Yurista, 2000) and may benefit from inhabiting a cooling water reservoir, like Newton Lake.

MATERIALS & METHODS

Newton Lake is used as a cooling water supply for the coal burning Newton Power Plant operated by Ameren (I.D.N.R., 2011; Ameren, 2011). This reservoir has a surface area of 718 ha with a maximum depth of approximately 12 m. Water is drawn from the eastern (cold) arm near the power plant and thermal effluent is released continuously near the water's surface at two locations in the northern half of the western (warm) arm (Fig. 1). Temperature of this effluent varies with energy output of the power plant. The cold arm, however, receives no heated effluent and provides a comparison with which to examine biotic responses to altered thermal regime in the warm arm. We consider the cold arm a reference condition at two scales; regionally the cold arm resembles other reservoirs with respect to climatic conditions that drive thermal patterns in temperate lentic systems, and within the reservoir basin, the cold arm and warm arm have similar physical (e.g., depth, surface area, shoreline structure) and watershed (high agricultural land use) characteristics. Water is released from Newton Lake via a small overflow dam, resulting in long residence time and an elongated lacustrine zone with little mixing of water between the two arms beyond the forebay.

To examine the relationship between temperature and cladoceran community patterns, four sample sites, two on each arm, were established in the limnetic zone of Newton Lake (Fig. 1). Labeled WAI and WAII on the warm arm and CAI and CAII on the cold arm, these sites are positioned at increasing distances from the heated discharge outflows. WAI is located adjacent to the southern discharge outflow while WAII is nearer to the forebay. CAI and CAII, located far from heated discharges, are intended to serve as reference sites. Overall, the spatial arrangement of sample sites allows for interpretation of biotic communities as they relate to temperature regime.

Sampling events occurred at biweekly intervals from Jul. 2003-Jul. 2004, except from Dec. through Feb. when only one sample was taken each month. During an event, maximum depth was determined to the nearest 0.1 m using a Hondex Digital Depth Sounder, and surface (0.5 m depth) water temperature was measured ([+ or -]0.1 C) with a YSI 85 multiple field meter. Beginning in spring 2004, additional temperature measurements were recorded at 2 m intervals from surface to substrate. These measurements were taken to determine the extent (i.e., depth) to which heated discharges alter vertical thermal patterns in the warm arm.

Zooplankton were collected at each of the four sample sites using a single vertical tow (Welch, 1948) from substrate to surface with an 80 [micro]m mesh conical plankton net with an 0.196 [m.sup.2] opening. All zooplankton samples were fixed using formalin-aceto-alcohol (Pennak, 1953) immediately upon collection and transported to the laboratory for later measurement of cladoceran community structure.

[FIGURE 1 OMITTED]

To assess community dynamics, cladoceran zooplankton were identified at 100x magnification to the lowest possible taxonomic level using Smith (2001), Thorp and Covich (2001), and Ward and Whipple (1918). Sample volumes were condensed in the laboratory (200-450 mL) to facilitate counting efficiency. For each sample, a 1 mL subsample was removed and placed into a Sedgwick-Rafter counting chamber for identification and enumeration of cladocerans. Upon completion, each subsample was returned to the original sample and the above process repeated four additional times. Cladoceran densities were calculated for each subsample using the formula outlined in Wetzel (2001).

Density, =

# of individuals in subsample * [sample volume(mL)/total subsample volume(mL)]/ area of net opening([m.sup.2]) * site depth(m)

For each sample, mean densities were determined by averaging the results obtained from density calculations of five subsamples.

Cladoceran community data were used to create a Bray-Curtis dissimilarity matrix for all pair-wise site comparisons on each sample date. Considered more accurate than other dissimilarity indices (Bloom, 1981), the Bray-Curtis index uses taxonomic composition and abundance to calculate a coefficient of dissimilarity between two communities, with higher coefficients indicating a greater degree of dissimilarity. Mean dissimilarity for the entire study duration was determined for each pair-wise comparison. Subsequently, nonmetric multi-dimensional scaling (NMDS) based on Bray-Curtis dissimilarity values (McCune and Mefford, 1999) calculated from each event and location combination was used to describe the relationship between community structure and temperature. NMDS enables low-dimensional, graphical representation of statistical distance between samples and is well suited for use with data which are non-normal (West et al., 2003; Pegg and McClelland, 2004). NMDS results were used to assess the relative statistical difference (dissimilarity) of each community as it relates to temperature.

RESULTS

Average surface water temperatures throughout the study duration were 28.6, 27.6, 22.1, and 22.7 C for WAI, WAII, CAI, and CAII, respectively. Although there is much overlap in temperature ranges, sites in the warm arm are characterized by higher temperatures in the summer months and maintenance of higher temperatures in the winter months (Fig. 2). Temperatures during summer in the warm arm remained above 30 C for approximately 16 wk and reached a maximum of 37.7 C, while the cold arm maintained 30 C or above for approximately 7 wk and reached a maximum of only 31.8 C. During seasonal transitions (spring and fall), warm arm sites cooled and warmed more rapidly than cold arm sites, and remained above 14 C during winter. Vertical temperature measurements revealed temperatures between arms varied most within the upper 4 m (Fig. 3). A weak thermocline appears to occur between 2-4 m, below which temperatures are similar between all four sites.

Seven cladoceran taxa, representing five families, were identified from the limnetic zone of Newton Lake. Cladoceran communities show similar patterns of development and decline at all four sites (Fig. 4). System-wide densities of cladocerans were lowest during both winter and summer (<3000/[m.sup.3]), while peak densities occurred in late fall and late spring (>4500/[m.sup.3]). Taxonomic richness also was relatively higher during fall mad spring (3-6 taxa), while during summer and winter richness remained at or below three taxa. No significant differences in cladoceran density between sites within the same arm were detected using t-tests with Benjamini and Hochberg False Discovery Rate corrections (1995; base P-value = 0.05). Density was greater in the warm arm than the cold arm during winter (P = 0.001), but the two arms were not significantly different during summer (P > 0.05).

Daphnia lumholtzi dominated the cladoceran community in Oct. and Nov. but also was present at much lower densities in Jun. Although temporal D. lumholtzi population patterns were similar in both arms, density maxima in the warm arm occur at a temperature between 20 and 23 C, while in the cold arm the maxima occur between 18 and 19 C. Maximum temperature at which D. lumholtzi was present also varied between the two arms (37.2 C in the warm arm and 31.6 C in the cold arm). Although D. lumholtzi was numerically and proportionally dominant most often at WAI (Fig. 4), this pattern was not consistently true and this species was not always more numerous within the warm arm than in the cold. At times from Dec. through May, Daphnia pulex comprised the majority of cladocerans in the community. Species of Daphnia were supplanted by Diaphanosoma ssp. from Jun. through Sept. and often was the only cladoceran present within the limnetic zooplankton community during that time. Bosmina spp., Aloha spp., Ilyocryptus spp., and Camptocercus spp. also were present in the community but never at high densities relative to more dominant taxa.

[FIGURE 2 OMITTED]

Mean Bray-Curtis dissimilarity values for each pair-wise cladoceran community comparison were separated into either within-arm or between-arm comparisons to determine general patterns of dissimilarity. Within-arm comparisons resulted in dissimilarity values of 0.355 [+ or -] 0.060 SD and 0.335 [+ or -] 0.044 for the warm arm and cold arm, respectively. Comparisons of communities between the two arms had relatively higher dissimilarity values (WAI-CAI = 0.590 [+ or -] 0.048; WAI-CAII = 0.493 [+ or -] 0.044; WAII-CAI = 0.565 [+ or -] 0.065; WAIICAII = 0.479 [+ or -] 0.049). We examined whether between-arm comparisons of dissimilarity values were statistically more dissimilar than within-arm comparisons using t-tests with Benjamini and Hochberg False Discovery Rate correction (base P value = 0.05), and in six of the eight possible comparisons, this pattern was true. The exceptions were WAI-CAII and WAII-CAII comparisons, which did not have significantly different dissimilarity values than the WAI-WAII comparison.

[FIGURE 3 OMITTED]

Statistical distance between each cladoceran community is shown by the distribution of points within the NMDS ordination space (Fig. 5). Each point in this space represents a community sampled at a particular site on one sample date. Points located far from one another are less similar than those that are more closely grouped. Once plotted, each point was placed into a category based on the temperature at which that sample was collected, and although arbitrary, these categories were selected based upon literature review pertaining to temperature tolerance or physiological response to temperature in zooplankton (McKee and Ebert, 1996; Achenbach and Lampert, 1997; Mitchell and Lampert, 2000; McKee et al., 2002). We also considered seasonal maximum and minimum temperatures in Newton Lake when defining thermal categories (Fig. 2). Those temperature categories are defined as low (5.014.9 C), moderate (15.0-24.9 C), and high (25.0-40.0 C). Cladoceran communities sampled at low temperatures are located in the upper portion of the ordination space, those sampled at high temperatures are located predominately in the lower half of the ordination space, while those sampled at moderate temperatures are located between the other two categories (Fig. 5).

[FIGURE 4 OMITTED]

DISCUSSION

Thermal effluent released from the Newton Power Plant creates distinct thermal regimes in each of the two arms of Newton Lake. Although overall patterns of increase and decline are similar, temperatures in the warm arm are consistently higher than those in the cold arm. Lake structure ensures little mixing of water between the two arms of the reservoir, minimizing directional flow from the warm arm into the cold arm.

Studies of other systems used as cooling water sources describe thermal patterns similar to those in Newton Lake. Temperatures near thermal discharges can be 10 C or higher over ambient (Marcy, 1971; Merriman and Thorpe, 1976; Larimore and Tranquilli, 1981), and a zone of increased temperature may extend several kilometers from the source (Laws, 1993). The largest temperature difference observed between the two arms in Newton Lake was nearly 12 C, rivaling the most extreme temperature alterations reported in other studies (Marcy, 1971; Laws, 1993).

[FIGURE 5 OMITTED]

In general, peak density and taxonomic richness of limnetic cladoceran communities in Newton Lake occur in spring and fall, while the converse occurs in winter and summer. Patterns of cladoceran population development and abundance are largely similar between the two reservoir arms, although some differences were observed. For instance, winter minimum in the warm arm is approximately 920 individuals/[m.sup.3] and 114 individuals/[m.sup.3] in the cold arm. Although no statistically significant differences were reveled due to large variation between sample periods, warm arm community dynamics suggest thermal effluent buffers cladocerans from cold temperatures in winter resulting in perpetuation of those species intolerant of cool temperatures in winter and producing greater cladoceran abundance when compared to the cold arm.

Temperature has been shown to have a large impact on seasonality and abundance of cladocerans by influencing time to maturity, brood size, and longevity (Wetzel, 2001). Although individual species vary, temperatures above 29-31 C have been shown to cause increased ephippial egg production (Threlkeld, 1979) and mortality in cladocerans (Carlson, 1973), resulting in an overall decline in community abundance. Peak densities of cladocerans in Newton Lake occur in both spring and fall when temperatures range from approximately 15-30 C. Populations develop in spring when temperatures increase into this range, while in fall they recover from low summer densities as temperatures decrease below 30 C.

Bray-Curtis dissimilarity values indicate cladoceran communities located in opposing arms of the reservoir are, in most cases, more dissimilar than those within the same arm. Dissimilarity values are greatest when comparing WAI and CAI communities and lowest between CAI and CAII communities. Furthermore, when considering only between arm comparisons, dissimilarity values are lowest between WAII and CAII communities. This pattern would suggest spatial differences in community structure relate to the abiotic properties of the sites themselves.

In addition to patterns suggested by inter-site dissimilarity value comparisons, points (representing cladoceran communities) within the ordination space produced by NMDS cluster according to the temperature at which they were sampled (Fig. 5). Interpretation of NMDS results suggests cladoceran community structure is largely related to temperature, either directly or indirectly. For instance, those communities sampled at cool temperatures were dominated by Daphnia pulex. On the other hand, warm temperature communities were dominated by Diaphanosoma spp. and overall community abundance and density was limited. Communities sampled at moderate temperatures occurred mostly in spring and fall when temperatures where rising or falling resulting in variable compositional structure where no single taxon dominated for any length of time.

Both comparisons of cladoceran community dissimilarity and NMDS analyses support our hypothesis that thermal effluents create differences in cladoceran communities between arms of Newton Lake. Atypical patterns of development and abundance (i.e., higher abundance in winter, dissimilar composition) in cladoceran communities present in the warm arm of Newton Lake likely result from alteration of temperature regime by thermal discharges released by the Newton Power Plant. Given the vertical stratification of temperature in Newton Lake (Fig. 3), cladocerans could escape high temperatures in the warm arm by migrating below the thermocline. However, cladocerans in Newton Lake spend a significant portion (more than half) of each day above the thermocline (Williams and Pederson, 2004) and, therefore, are subjected to site-specific thermal patterns.

Although temperature often is recognized as a primary driver, studies of zooplankton dynamics in temperate systems conflict with respect to causation of seasonal patterns in development and abundance. Peak cladoceran community density and taxonomic richness often occur in spring and into early summer (Pennak, 1953; Threlkeld, 1979; Waite, 1981), although this pattern is system dependant. Others attribute a mid-summer decline to intense predation by planktivores (Luecke et al., 1990; Mehner, 2000; Jeppesen et al., 2004). However, several observations suggest limited depredation by planktivores and a consequential development and persistence of cladoceran populations in Newton Lake during spring and fall. A fish diet study in Newton Lake concurrent to ours (unpubl. data) found gizzard shad were the dominant species present in the limnetic zone and that this species was primarily a detritivore. That study also determined juvenile largemouth bass and bluegill were the dominant planktivores in Newton Lake and that these species began a dietary shift to larger prey items (i.e., insects and fish) in late May. This observance of limited planktivory by fish in summer contrasts other studies that have implicated depredation as the primary cause of declining cladoceran abundance in systems containing vertebrate predators (King and Greenwood, 1992). One possible explanation for the persistence of cladocerans in Newton Lake is that these organisms undergo diel vertical migration and move below the euphotic zone (Williams and Pederson, 2004) allowing them to escape visual predators (Zaret, 1980; Wetzel, 2001) during the daytime. Furthermore, lack of planktivores (comprised mainly of young-of-the-year game species) in the limnetic zone of Newton Lake may spatially separate cladocerans from potential fish predators. Consequently, low summer density of cladocerans in Newton Lake is most likely a result of temperature rather than depredation.

Daphnia lumholtzi was present at high densities in Newton Lake during periods when densities of other cladoceran taxa were limited, although this pattern was system-wide and not restricted only to the warm arm. It is unknown whether this pattern occurs because D. lumholtzi is more tolerant of high temperatures (resulting from either cooling water effluent or natural summer heating) and, therefore, gains a competitive advantage during warm periods, or because differential planktivory by fish is thought to occur when D. lumholtzi and native Daphnia are both present in zooplankton communities (Kolar et al., 1997). However, we suggest these patterns result from the former as few planktivores exist in Newton Lake in the limnetic zone (i.e., gizzard shad are the primary inhabitant of open water habitat in Newton Lake).

Much research effort has been directed towards the ecological significance of Daphnia lumholtzi as it is an exotic species which has spread across a large portion of the United States during the past 25 y (Havel and Hebert, 1993; Lemke et al., 2003). Its pronounced morphological features are theorized to deter predation by gape-limited planktivores (Swaffer and O'Brien, 1996; Kolar et al., 1997), although its utility as a prey item has been debated (Lemke et al., 2003; Metzke and Pederson, 2006). In Newton Lake it appears D. lumholtzi will have little impact to feeding by planktivorous fish as temporal distance exists between these two taxonomic groups (i.e., D. lumholtzi is most abundant in fall after planktivorous fish have undergone a dietary shift to insectivory or piscivory).

This study suggests altered thermal regimes created by heated effluents can create atypical cladoceran population dynamics and also can alter interactions between planktivorous fish and these cladocerans. As Illinois contains at least 37 power plants (I.E.P.A., 2011), presumably all of which discharge heated effluent into a waterbody, results from this study could offer additional information pertaining to ecological impacts of these facilities within the state and within the region. Many studies focus on vertebrates in cooling water reservoirs, but conclusions present here offer insight into the manner in which thermal discharges impact lower trophic levels. Furthermore, given the widespread and expanding range of Daphnia lumholtzi in the United States, the relationship between this species and planktivorous fish observed in this study could aid in understanding its ecological impact, especially in relation to its value as a prey item.

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SUBMITTED 14 JANUARY 2010

ACCEPTED 25 OCTOBER 2011

B. A. METZKE

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AND

C. L. PEDERSON

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