Combined Impacts of Algal Density and Copepod Predation on the Community of Small-Sized Zooplankton

119 ［Japanese Journal of Water Treatment Biology Vol.53 No.4 119－128 2017］ Combined Impacts of Algal Density and Copepod Predation on the Community of Small-Sized Zooplankton TAKAMARU NAGATA1* and TAKAYUKI HANAZATO2 1Lake Biwa Environmental Research Institute/5－34 Yanagasaki, Otsu, Shiga 520－0022, Japan 2Institute of Mountain Science Shinshu University, Division of Science for Inland Water Environment /5－2－4 Kogandori, Suwa, Nagano 392－0027, Japan Abstract We assessed the combined impacts on the community of small-sized herbivorous zooplankton using mesocosms, in which their populations were exposed to copepod Mesocyclops pehpeiensis predation and/or food shortage. The densities of the small-sized zooplankton were depressed in the tanks with M. pehpeiensis and a low algal density, probably because the prey populations were unable to compensate for the population loss by copepod predation. In contrast, most of the herbivorous zooplankton, especially rotifers, established large populations, despite their exposure to predation in the tanks with high algal density. This might be due to their high reproductivity under the conditions. In addition, the release from competitive pressure by cladocerans, whose populations were suppressed by intensive predation by M. pehpeiensis, would be favorable for the rotifers. Our experimental findings may explain the herbivorous zooplankton community structure in the shallow eutrophic lakes inhabited by abundant copepods. The results may become valuable information for reducing biological interference with rotifers within zooplankton assemblages, leading to effective use of the animals for water treatment systems. Keywords: community structure, copepod predation, food abundance, small sized zooplankton ( ) INRODUCTION algal, bacterial and detrital resources , which governs the reproduction of zoo- Predacious copepods often appear abun- plankton populations5)－8). Herbivorous zoo- dantly in lakes, and impose high predation plankters compete with one another for food pressure on small-sized herbivorous zoo- resources because most of them utilize plankton such as rotifers1). Copepods prey particles within a common size range of up effectively on small-sized, soft-bodied, and to 25 µm (4－17 µm for rotifers8)－11); 1－25 µm non-spined prey zooplankton species2),3). This for cladocerans7),12),13). Many studies have is probably the main factor accounting for shown that population densities of the small the phenomenon that, in the field, copepods zooplankters, such as rotifers, decline in the exert different degrees of predation pressure presence of large cladocerans such as on each of herbivorous zooplankton popu- Daphnia9),11). Cladocerans generally have lations, and that they alter species composition large body and food competition advantage of the prey community1),4). over rotifers. This advantage is attributed A factor other than predation that controls mainly to the fact that large zooplankters population dynamics of small-sized her- have a high clearance rate and tolerance to bivorous zooplankton is food abundance starvation9). In addition, the wider size range ＊ Corresponding author Phone number: +81 77 526 4800 E-mail: [email protected] 120 Japanese J. Wat. Treat. Biol. Vol.53 No.4 of food particles available to cladocerans, in 100 L, diameter 40 cm, height 73 cm) were comparison with rotifers, may contribute to used for the experiment. To establish a the better food competitiveness. zooplankton community in each tank, the The community structures of herbivorous bottom sediments of the eutrophic Lake zooplankton would be controlled in a complex Suwa, Japan, including resting eggs of manner by food resources and invertebrate various zooplankton species, was collected at predation. It is expected that the populations the center of the lake (6 m depth; 36°2’N, of small-sized zooplankton species, especially 138°5’E) with an Ekman grab sampler. About rotifers, which dominate zooplankton com- 1.5 kg of sediment was placed in each tank, munities in eutrophic lakes (generally fish- which was then filled with 80 L of tap water, abundant lakes) are imposed strong effect by and the experiment was started (day 0). All the combined effects. Predaceous copepods of the tanks were kept in the laboratory in a play significant role in determining com- controlled environment (temperature 20℃, munity structures of herbivorous zooplankton photoperiod 16－h light/8－h dark). On day 10, even in such lakes1),14) because they are able the mesocosm tanks were divided into two to maintain its populations by their escape groups of six tanks each: low-food tanks and response, diel vertical migration which is high-food tanks (Fig. 1). On the same day, to effective to reduce predation pressure by create different food levels for the herbivorous fish15). Chang and Hanazato16) have performed zooplankton in the tanks, we added the a community-level laboratory experiment to green alga Chlorella vulgaris (purchased evaluate the combined impacts of food from Chlorella Industry Co. Ltd., Fukuoka, abundance and copepod predation on zoo- Japan) to the low-food and high-food tanks to plankton assemblage. However, they were create algal densities of approximately 1×103 unable to evaluate the combined impact on cells/mL and 1×105 cells/mL, respectively. planktonic rotifer species since the densities Algae were added repeatedly at 1－ or 2－day of such species were low, but the benthic intervals, thereafter. On day 19, 120 adults species Lepadella sp. was abundant during of Mesocyclops pehpeiensis (body length, > the experiment. Thus, we tried to assess the 1000 µm) were introduced into each of three combined impacts of algal concentration and low-food tanks and three high-food tanks copepod predation on small-sized herbivorous (low-food Mesocyclops tanks, high-food zooplankton, particularly planktonic rotifer Mesocyclops tanks, respectively). The pred- populations in a community-level experiment. ators were obtained from Lake Suwa one month before the start of the experiment and MATERIALS AND METHODS maintained in the laboratory with a Twelve cylindrical plastic tanks (volume zooplankton assemblage collected from the Figure 1 The experimental protocol. Combined Impacts of Algal Density and Copepod Predation on the Community of Small-Sized Zooplankton 121 lake. No invertebrate predators were early and late copepodite stages are generally introduced into the remaining three low-food known to be herbivorous and omnivorous, tanks and three high-food tanks, which respectively, and therefore the latter stage is served as controls (low-food control tanks, considered to be when M. pehpeiensis feeds high-food control tanks, respectively). The mainly on herbivorous crustaceans and experiment was terminated on day 27. rotifers. Water temperature and phytoplankton The data sets were analyzed using 2 × 2 biomass indicated by chlorophyll a (chl. a) in factorial ANOVA with the difference in algal the tanks were measured at 1－ or 2－day density and the introduction of M. pehpeiensis intervals. Water temperature was measured as the main factors, and day as repeated with a thermister (YSI 55 dissolved oxygen measures. The differences in density of meter; YSI Co., Ohio, USA). For measurement zooplankton species among tanks subjected of chl. a, 100 mL of water was collected from to different treatments were analyzed each tank and filtered through a Whatman statistically with two-way repeated-measures GF/C filter, and then chl. a in the particles ANOVA using StatView ver.5 (SAS Institute that remained on the filter was measured Inc., Cary, NC, USA). using methyl alcohol and fluorescence 17) RESULTS according to Marker et al . Zooplankton were sampled from all the Water temperature did not differ markedly tanks on day 16 and at 1－ or 2－day intervals between the control and the Mesocyclops thereafter. To collect zooplankton samples, tanks under the respective food conditions the water column (3.0 L in volume) from the (Fig. 2). Under the high-food conditions, the surface to 32 cm depth (approximately 20 cm values of chl. a tended to be lower in the above the bottom sediment) in the tanks was control tanks than those in the Mesocyclops collected using a column sampler (diameter 5 tanks, but the difference was not significant cm, length 50 cm) with a hydraulically (repeated-measures ANOVA, treatment: F = operated bottom flap. Since some zooplankters 1.211, P = 0.2820). The values of chl. a in the might have been distributed unequally in the low-food tanks were very low and were tanks, the water was gently mixed with a therefore hardly determined during most of wooden rod before sampling to make their the experimental period. distribution uniform. Zooplankton were then It was impossible to completely exclude M. collected by filtering the sampled water pehpeiensis from the control tanks, because through a 40－µm mesh net, and preserved in their resting stages were contained in the sugar-formalin containing a final concen- bottom sediment, and hence some copepods tration of 4% formalin18). The mesh size (40 appeared in the tanks (Fig. 3). However, the µm) of the net was sufficiently smaller than densities of their early and late copepodite the body width of small species such as the stages were higher in the Mesocyclops tanks rotifers Filinia and Polyarthra8),19). Therefore, than in the control tanks during the all the crustacean and rotifer individuals experiment (Table 1). In the experiment, (except eggs) should have been collected with unexpected invertebrate predators such as the net. The water that had filtered through Leptodora kindtii (except for M. pehpeiensis) the net was returned to the tank to reduce did not appeared in the mesocosm tanks. water loss. Zooplankton in the samples were Three cladoceran (Bosmina longirostris, B. identified to species or genus level and fatalis and Bosminopsis deitersi) and four counted with a microscope at ×100 magnifi- rotifer species (Brachionus angularis, Filinia cation. For the predator M. pehpeiensis, the longiseta, Hexarthra mira, and Keratella body lengths of individuals were measured valga) appeared commonly in the mesocosm and divided into two life stages according to tanks (Figs.

Load more