Temperature Effects on Body Size of Freshwater Crustacean Zooplankton from 4 Greenland to the Tropics 5 6 Karl E
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1 Accepted by Hydrobiologia: 14/7/014 2 3 Temperature effects on body size of freshwater crustacean zooplankton from 4 Greenland to the tropics 5 6 Karl E. Havens1, Ricardo Motta Pinto-Coelho2, Meryem Beklioğlu3,4, Kirsten S. 7 Christoffersen5, Erik Jeppesen6,7, Torben L. Lauridsen6,7, Asit Mazumder8, Ginette Méthot9, 8 Bernadette Pinel Alloul9, U. Nihan Tavşanoğlu3, Şeyda Erdoğan3 and Jacobus Vijverberg10 9 10 1 University of Florida and Florida Sea Grant, Gainesville, FL 32608 USA 11 12 2 Departamento de Biologia General, Universidade Federal de Minas Gerias, 31270-901 13 Belo Horizonte, Brazil 14 15 3 Middle East Technical University, Department of Biological Sciences, Ankara, Turkey 16 17 4 Kemal Kurdaş Ecological Research and Training Stations, Lake Eymir, Middle East 18 Technical University, Ankara, Turkey 19 20 5 Institute of Biology, Universitetsparken 4, Copenhagen, Denmark 21 22 6 Department of Plant Biology, University of Aarhus, Nordlandsvej 68, Risskov, Denmark 23 24 7 Sino-Danish Centre for Education and Research, Beijing, China 25 26 8 Department of Biology, University of Victoria, P.O. Box 3020, Stn. CSC, Victoria, British 27 Columbia V8W 3N5, Canada 28 29 9 Groupe de Recherche Interuniversitaire en Limnologie, Départment de Sciences 30 biologiques, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, QC, Canada 31 32 10 Netherlands Institute of Ecology, Droevendaalsesteeg 10, 6708 PB Wageningen, The 33 Netherlands 34 35 Corresponding author and contact information: 36 E-mail: [email protected] 37 Telephone: (352) 392-5870 38 Fax: (352) 392-5113 39 1 40 Abstract The body size of zooplankton has substantive effects on the function of aquatic 41 food webs because size affects the ability of zooplankton to: exploit food resources, 42 effectively clear the water of algae, and serve as a food source for fish. A variety of factors 43 may affect size, and earlier studies indicate that water temperature may be a particularly 44 important variable. Here we tested the hypothesis that the body size of cladocerans, 45 calanoids and cyclopoids declines with increasing water temperature, a response 46 documented in an earlier study that considered only cladoceran zooplankton. We tested 47 the hypothesis by comparing body size data that were available from prior studies of lakes 48 ranging from 6 to 74o latitude and encompassing a temperature range of 2 to 30oC. 49 Cladoceran body size declined with temperature, however, the trend was just marginally 50 significant (p = 0.01). The decline in size was significant (p = 0.05) for cyclopoids. In both 51 cases, there was considerably more variation around the regression than previously 52 observed suggesting that other variables such as fish predation played an important role in 53 affecting size. Calanoid body size was unrelated to temperature. In contrast with 54 cladocerans and cyclopoids, perhaps calanoid body size is not metabolically constrained by 55 temperature or is differently affected by changes in fish predation occurring with 56 increasing temperature. The unexpected result for calanoids requires further investigation. 57 58 Keywords zooplankton size; latitudinal patterns; global comparison 59 60 2 61 Introduction 62 Body size of zooplankton is a highly important attribute because of the influence it has on 63 bioenergetics in aquatic food webs. Body size affects the manner in which the zooplankton 64 interacts with its resources. Large-bodied zooplankton have higher grazing rates, they 65 graze a wider size range of food items, and therefore have greater top-down effects on 66 resources than small-bodied zooplankton (Carpenter & Kitchell, 1988). Likewise body size 67 affects the availability and antipredator responses of zooplankton to both invertebrate and 68 vertebrate predators (Zaret, 1980; Gelinas et al., 2007), and the relationship between 69 phytoplankton biomass and phosphorus in lakes (Mazumder, 1994a; b and see the review 70 by Meerhoff et al., 2012). 71 In warm water lakes the size of zooplankton generally is smaller than of those in cold 72 water lakes. The reasons for this trend have been debated, yet so far, answers are 73 inconclusive. When considered alone and independent of other factors, higher temperature is 74 expected to result in smaller animals because higher temperature shortens generation time 75 (Gillooly, 2000) resulting in smaller adult body size (Geiling & Campbell, 1972; Angilletta & 76 Dunham, 2003). This phenomenon is not restricted to zooplankton. Atkinson (1994) 77 conducted a review and found that >80% of ectothermic species studied in laboratory 78 experiments displayed faster growth and smaller adult size at higher temperatures, and in 79 general, animals in colder climates are larger as adults then conspecifics at colder 80 temperatures (Ashton, 2001). 81 Larger zooplankton also may be more susceptible to thermal stress at high 82 temperatures (Moore et al., 1996). However, changes in temperature also can have indirect 83 effects on the food web, and these could be of greater importance than direct effects in 3 84 determining zooplankton body size. Hart & Bychek (2011) reviewed the factors affecting 85 body size and concluded that ‘direct thermal effects on body size are commonly confounded 86 by other environmental factors that are related directly or indirectly to temperature’ and 87 that ‘predation undoubtedly has an over-riding influence on body size selection.’ 88 One prior study examined the relationship of body size in freshwater cladoceran 89 zooplankton with latitude and temperature (Gillooly & Dodson, 2000) using data from 90 1,100 lakes between 7 and 78o latitude in North, Central and South America. The authors 91 found that temperature alone explained over 90% of variation in cladoceran body size. In 92 that study the authors did not use direct measurements of size. They took species lists from 93 the literature and estimated size based on assumptions including invariant size within 94 species and even distribution of density among species in lakes. This makes our study 95 unique, in being the first to address the relationship between size and temperature with 96 actual measurements of density and biomass of cladocerans, and the first to broaden the 97 assessment to include cyclopoids and calanoids. 98 We obtained crustacean zooplankton data from published studies conducted in 99 Brazil, Canada (British Columbia and Ontario), Ethiopia, Greenland, Turkey and the USA 100 (Alaska and Florida). All datasets had direct measurements of density, and biomasses were 101 calculated from body lengths using published and well-accepted allometric methods. We 102 calculated the mean body size (g dry weight) of cladocerans, calanoids and cyclopoids in 103 each region by dividing total biomass by total density in each of these major taxonomic 104 groups. We tested the hypothesis that body size of cladocerans, cyclopoids and calanoids 105 declines with temperature across the broad latitudinal gradient of our study lakes. 106 4 107 Methods 108 Study Lakes 109 We used pre-existing data from lakes representing a wide range of nutrient concentrations, 110 six countries and several climactic regions (Fig. 1, Table 1). Late summer samples were 111 used (August-September in Canada, United States and Turkey, and September in 112 Greenland), because only that period of time was sampled in synoptic surveys in Greenland 113 and Turkey and so that data would be comparable in regard to the time of full development 114 of crustaceans and highest fish predation (Jeppesen et al., 1997). In Ethiopia and Brazil, 115 data were taken from the dry season (November to January in Ethiopia, July-August in 116 Brazil), which also are times of high fish predation (Vijverberg et al., 2014). During the 117 rainy season (July-September) in N.E. Africa and in South America, short water residence 118 times flush zooplankton from lakes and reservoirs, making it an unsuitable time for 119 studying zooplankton. 120 121 Sampling Methods 122 Detailed sampling methods are provided in published papers dealing with other aspects of 123 the plankton: for Greenland (Jeppesen et al., 2001); Ontario, Florida and Brazil (Pinto- 124 Coelho et al., 2005); British Columbia (Kainz et al., 2004); and Africa (Vijverberg et al., 125 2014). Therefore, only brief descriptions of methods are provided here. 126 Sampling methods in all lakes except Turkey collected animals from the entire water 127 column. In Greenland, mid-lake depth-integrated water samples (20-25L) were taken with 128 a Patalas sampler. Of the pooled sample, a 15-20 L subsample was filtered on a 20 m filter. 129 In Turkey, a 40 L mid-lake integrated water sample through the entire mixed layer was 5 130 taken with a KC Denmark Water Sampler (3.5 L), and 20 L was filtered through a 20 μm 131 filter. In Ontario, zooplankton was sampled from the entire water column at the deepest 132 site in the lakes by vertical tows with a 53 m mesh conical plankton net. In British 133 Columbia and Alaska, vertical tows of a 64 m mesh conical net were taken from 1 m above 134 the sediments to the surface at the deepest location in the lakes. In Florida, Ethiopia and 135 Brazil, single vertical tows of the entire water column were done with conical nets with 136 153, 150 and 90 m mesh, respectively. In Ethiopia, water column samples were collected 137 from three open-water sites, whereas in Brazil, water column samples were taken from 11 138 sites in Furnas Reservoir and 2 sites in Ibirité Reservoir (Pinto-Coelho et al. 2005). 139 Samples from Alaska, Canada, Florida, Ethiopia and Brazil were preserved in 10% 140 formalin. In Greenland and Turkey, samples were preserved in 4% Lugol’s. Crustacean 141 zooplankton was enumerated at 40, 50 or 100x magnification, and 25-50 individuals 142 (where possible) of each species were measured with an ocular micrometer to the nearest 143 25 m.