Abstract: Time trends in abundance, body size, species richness, and species composition indicate that crustacean zooplankton communities of southern Canadian Shield lakes changed between 1980 and 2003. Total abundance did not decline despite reductions in total phosphorus, but all other metrics changed. Species richness declined in Harp Lake (Ontario, Canada) following its Bythotrephes invasion, but richness increased in three other lakes. Average cladoceran body length increased from 0.6 to 1.0 mm in seven of the lakes, as larger-bodied taxa replaced smaller ones. Correlations with water quality and fish metrics suggest that cladoceran size increases were attributable to many factors: a decline in food availability following declining phosphorus levels increasing the competitive advantage of larger herbivores, a decline in acidity favouring the larger, acid-sensitive daphniids, and reduced risk of planktivory linked to a rise in dissolved organic carbon levels and changes in predation regimes. Zooplankton communities on the Canadian Shield are changing, and these changes are best viewed in a multiple-stressor context. Key anthropogenic stressors have also changed and may do so again if Ca concentrations continue to decline.
Resume : Les tendances temporelles de l'abondance, de la taille corporelle, de la richesse specifique et de la composition en especes indiquent que les communautes de crustaces zooplanctoniques dans les lacs du sud du Bouclier laurentien ont change entre 1980 et 2003. L'abondance totale n'a pas diminue malgre les reductions du phosphore total, mais toutes les autres metriques ont ete modifiees. La richesse specifique au lac Harp (Ontario, Canada) a decline apres l'invasion de Bythotrephes, mais la richesse a augmente dans trois autres lacs. La longueur corporelle moyenne des cladoceres s'est accrue de 0,6 a 1,0 mm dans sept des lacs, car des taxons a corps plus grand ont remplace les taxons a taille plus petite. Les correlations avec la qualite de l'eau et les metriques des poissons indiquent que l'augmentation de taille des cladoceres s'explique par plusieurs facteurs : une baisse de la disponibilite de la nourriture a cause du declin des concentrations de phosphore, ce qui a accru l'avantage competitif des herbivores plus grands, une baisse de l'acidite qui a favorise les daphniides sensibles a l'acidite qui sont de plus grande taille, ainsi qu'une diminution du risque de planctonophagie reliee a l'accroissement des concentrations de carbone organique dissous et a des modifications des regimes de predation. Les communautes zooplanctoniques sur le Bouclier laurentien sont en train de changer et ces modifications s'expliquent mieux dans le contexte d'agents multiples de stress. Les sources anthropiques majeures de stress ont aussi change et peuvent encore le faire si les concentrations de Ca continuent a decliner.
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Introduction
Aquatic ecologists have long been interested in the zooplankton of temperate soft-water lakes such as those of the Canadian Precambrian Shield (Klugh 1921; Adamstone 1928). Given the extent of past work, one might think that we understand the regulators of these zooplankton communities. Certainly, many of the natural regulators are known. At the largest spatial scale, latitude, glacial history, and regional bedrock geology are each important (Pinel-Alloul et al. 1979), given their respective effects on lake thermal regimes (Patalas 1990), community assembly (Carter et al. 1980; Stemberger 1995), and water chemistry (Tessier and Horwitz 1990; Hessen et al. 1995; Waervagen et al. 2002). Smaller-scale, landscape patterns are also important. Larger lakes, with their greater probability of colonist introduction and greater habitat diversity, support more taxa (Dodson 1992; Keller and Conlon 1994). Landscape position influences water chemistry (Quinlan et al. 2003) and connections to colonist, competitor, and predator sources (Cottenie et al. 2001; Beisner et al. 2006). Watershed features also influence the supply of macronutrients (eg. N, P, and C) to lakes, and this nutrient supply influences both total (Yan 1986) or specific size fractions (Finlay et al. 2007) of zooplankton biomass and the competitive interactions of species with different absolute (Gliwicz 1990) and relative nutrient requirements (Sterner et al. 1997; Elser et al. 1998). The dissolved organic fraction of the carbon supply (DOC) affects the reactive distance of visual predators (Carter et al. 1983; Wissel et al. 2003), lake acidity, and microbial food availability (Moore et al. 1994). Finally, among the natural regulators, zooplankton may be influenced by the size, shape, and biochemical composition of their algal prey (Park et al. 2002; Ghadouani et al. 2003; Hampton et al. 2006), by competition with other zooplankton (Allan 1973), by parasites (Yan and Larsson 1988; Ebert 2005), and by both vertebrate and invertebrate predators (e.g., Magnan 1988; Yan et al. 2001a).
In addition to these natural regulators, many anthropogenic stressors have also influenced Canadian Shield zooplankton. Historically, the common stressors were local ones, i.e., land clearing, agriculture, and forestry (Patoine et al. 2002), shoreline development (Dillon and Molot 1996), water level regulation (Quinlan and Smol 2001), and fish introductions (St. Jacques et al. 2005). More recently, anthropogenic stressors have regional or even larger-scale signatures. For example, increased ultraviolet (UV) irradiance affects the behaviour, distribution, and perhaps the composition of zooplankton communities in clear-water lakes (Williamson et al. 2001; Persaud and Yan 2003; Leech et al. 2005), but the majority of Shield lakes are probably not UV-transparent enough to be much affected (Molot et al. 2004). In contrast, acid rain has disturbed zooplankton in thousands of lakes in eastern Canada (e.g., Carter et al. 1986; Pinel Alloul et al. 1990; Havens et al. 1993), and near smelters, the damage has been compounded by metal toxicity (Yan and Strus 1980). Regionally coherent, annual fluctuations in their abundance indicate that Canadian Shield zooplankton are also responding to a changing climate (Rusak et al. 1999). Finally, Canadian Shield zooplankton are also now affected by the redistribution of native predators (Vander Zanden et al. 2004b) and the introduction of nonnative ones (Yan et al. 2002). Despite this body of past understanding, built on both large (e.g., Carter et al. 1980; Keller and Pitblado 1989; Patalas 1990) and more regional synoptic surveys (e.g., Carter 1971; Patalas 1971; Pinel-Alloul et al. 1990), long-term observational studies (e.g., Yan and Pawson 1997; Keller et al. 2002; Olden et al. 2006), and whole-lake experiments (Keller et al. 1992; Elser et al. 1998; Yan et al. 2001a), it is again time to examine long-term trends in the crustacean zooplankton fauna of the Shield, because zooplankton are now facing both new stressors with unknown effects (e.g., Ca decline, Jeziorski and Yan 2006) and, more importantly, combinations of abiotic and biotic stressors that are unique in their postglacial experience (e.g., Strecker and Arnott 2005). Long-term data sets may be quite useful for teasing out multiple-stressor interactions (Hampton et al. 2006) and for comparing the magnitude of interlake and interannual variability (Olden et al. 2006). Here, we examine the 1980 to 2003 crustacean zooplankton records from the eight, long-term study lakes (Table 1) of the Ontario Ministry of the Environment's Dorset Environmental Science Centre (DESC) in south-central Ontario, Canada, to determine (i) if the magnitude of interannual variability in the communities differs among lakes, (ii) if there are long-term trends in community abundance, species richness, composition, and (or) size structure, (iii) if key changes are lake-specific or regional in scale, and (iv) which water quality changes, or combinations of changes, are correlated with the key observed trends. We focus on water quality correlations because climate drivers are considered elsewhere (Rusak et al. 1999).
Given that ice-free season length (Futter 2003; Keller 2007), water quality (Dillon et al. 2007), and predator assemblages (Boudreau and Yan 2003; Vander Zanden et al. 2004a) have all changed for many lakes on the south-central Shield and given that each may alter zooplankton communities (Chen and Folt 2002; Yan et al. 2002; Olden et al. 2006), we expected that the zooplankton communities of all of our long-term study lakes would have changed. Total phosphorus (TP) levels declined appreciably and pH rose in all but one of the lakes (Table 2). Because the magnitude of interannual variability in water quality differed among lakes, we predicted that the magnitude of interannual variability in the zooplankton metrics (objective 1) would also differ. We predicted a decrease in total zooplankton abundance, given the declining TP trends (Table 2) and the positive correlation between nutrients and zooplankton standing stocks observed in spatial surveys in the region (Yan 1986). Except for Harp Lake, with its Bythotrephes invasion (Yan and Pawson 1997), we predicted an increase over time in species richness, given the reduction in acidity in the lakes (Table 2) and the commonly observed, positive correlation between pH and crustacean species richness (Locke 1992). We predicted increases over time in the relative abundances of larger vs. smaller cladoceran species, leading to an increase in mean body size of the cladoceran assemblage, for a number of reasons. TP levels are falling, and larger herbivorous zooplankton should outcompete smaller ones as food resources fall (Gliwicz 1990). Chromophoric DOC levels have increased over time in several of the lakes (Keller et al. 2008), decreasing water clarity and lowering the relative risk of the larger taxa to visual predators. Acidity has also declined, favouring the acid-sensitive daphniids (Havens et al. 1993), which, excluding Daphnia ambigua, are among the larger pelagic Cladocera. Smallmouth bass (Micropterus dolomieu; Vander Zanden et al. 2004a) and Bythotrephes longimanus (Yan and Pawson 1997) are appearing in area lakes, and both introductions commonly lead to increases in mean cladoceran body size (Yan and Pawson 1997; Yan et al. 2001a). Fish are particularly important regulators of zooplankton community size structure in low productivity lakes such as these (Finlay et al. 2007). Finally, although Ca levels are falling in most of the lakes, we do not believe they have declined enough as yet to harm the larger, more Carich daphniids (Jeziorski and Yan 2006; Ashforth and Yan 2008).
Materials and methods
The study lakes
Blue Chalk, Chub, Crosson, Dickie, Harp, Heney, Plastic, and Red Chalk lakes are the focus of this study. The lakes were selected to vary widely in sensitivity to acid inputs and in proportion of the theoretical total phosphorus load that was attributed to inputs from shoreline dwellings (Dillon and Molot 1996). The early 1990s introduction of the nonindigenous zooplanktivore Bythotrephes longimanus to Harp Lake (Yan and Pawson 1997) added an unplanned stressor to the planned pH and TP gradients of the lakes (see Table 1) and left only Blue Chalk and Red Chalk lakes as reference systems, i.e., as clear-water, dimictic lakes that were selected to be unaffected by shoreline development, acid rain, or introduced zooplanktivores.
The water quality of the lakes has changed significantly over the 24-year study period. Levels of Ca declined in seven of the eight lakes (Table 2; Watmough and Aherne 2008), increasing only in Dickie Lake, likely associated with dust control treatment of roads in its watershed (Table 2). Levels of pH increased in all but one of the lakes over the entire study period in response to reductions in acid deposition (Table 2; Dillon et al. 2007). In Red Chalk, Blue Chalk, and Harp lakes, levels of pH were always >6, i.e., above the commonly accepted threshold for zooplankton damage (Havens et al. 1993; Holt and Yan 2003). Levels of TP declined substantially in all the lakes over time (Table 2), and algal composition also changed, with colonial chrysophytes replacing diatoms as the dominant taxa in several of the lakes (Paterson et al. 2008).
Zooplankton sampling and analysis
Between 1980 and 2003, 1635 samples were collected and enumerated from the lakes. Animals were collected between 0800 and 1600 as a single bathymetrically weighted, composite sample from a station located over the point of maximum depth in each lake. The composite was formed by combining the contents of four to seven vertical hauls taken from predetermined depths to the surface using a 12.5 cm diameter, 80 ?m mesh, high efficiency (McQueen and Yan 1993), metered, tow net. The lengths and numbers of hauls were set so that lake depth strata contributed to the composite sample in approximate proportion to their volumes. Samples were preserved immediately after collection with a buffered sucrose formalin solution. Except for Harp, Plastic, and Dickie lakes, all lakes were sampled on a monthly basis during the ice-free season after 1980. Harp and Plastic lakes were sampled on a fortnightly basis. Dickie Lake was sampled monthly until 1988, then fortnightly thereafter. Such differences in sampling frequency have very minor effects on ice-free season averages of zooplankton community metrics (Yan 1986; J. Rusak, personal observation).
In the laboratory, all animals were identified and enumerated by either Bill or Dee Geiling (Limnoservices Inc., Lansdowne, Ontario, Canada). A minimum of 250 individuals were identified and enumerated in each sample, and body lengths of video images of all counted animals were measured (Allen et al. 1994) from the top of the head to the base of the tail spine or mucron in Cladocera or to the base of caudal ramus in Copepoda. Cladocera and adult Copepoda were identified to the species level, while immature copepodids were identified to the level of …
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