Takamaru Nagata

Different Predation Impacts of Two Cyclopoid Species on a Small-sized Zooplankton Community: An Experimental Analysis with Mesocosms

We used mesocosms to analyze predation impacts on the prey populations and prey community structures by two cyclopoid copepod species, the larger Mesocyclops pehpeiensis and the smaller Thermocyclops taihokuensis, who coexist with small-sized herbivorous zooplankton species in a fish-abundant lake. The overall predation impact on the prey populations was stronger for Mesocyclops than for Thermocyclops. Mesocyclops had a strong and less selective impact on the rotifer community but a selective impact on the crustaceans. In contrast, Thermocyclops had a selective predation impact on rotifers but a weak and less selective impact on the crustacean community. As a result, the former predator reduced the diversity of the crustacean community but not the rotifer community, while the latter had an opposite impact on the diversities of the two communities. It has been suggested that fish induce development of a zooplankton community dominated by the small-sized zooplankton species in fish-abundant lakes. Our results demonstrated that cyclopoid copepods altered species composition and diversity of the small-sized zooplankton community in such lakes. Thus, the results have given an important suggestion on the role of the invertebrate predator cyclopoid copepods, which often coexist with fish, that they determine population dynamics and community structures of small-sized zooplankton in fish-abundant lakes.

Direct and indirect impacts of predation by fish on the zooplankton community: an experimental analysis using tanks

The impact of Pseudorasbora parva, a common zooplanktivorous fish species in Japan, on a zooplankton community was analyzed in experimental tanks, half of which were stocked with the fish. Different zooplankton species showed different responses to the introduction of the fish. In the presence of the fish, the populations of the large cladoceran Ceriodaphnia and the predatory copepod Mesocyclops were reduced, but the population of the herbivorous copepod Eodiaptomus and the small cladocerans Bosmina fatalis and Bosminopsis deitersi increased relative to the controls. The increase of Mesocyclops seen in the control tanks might have suppressed the populations of the small cladocerans, which are vulnerable to invertebrate predation. The results suggest that the population densities of the large prey items preferred by the fish, Ceriodaphnia and Mesocyclops, were controlled directly by fish predation, but the population densities of the smaller and less preferred zooplankton were controlled indirectly through the food-web cascade.

The Predation Impact of Larval Pseudorasbora parva (Cyprinidae) on Zooplankton: a Mesocosm Experiment

ABSTRACT The impact of larvae of the fish Pseudorasbora parva on a zooplankton community was studied by adding the fish to tanks housing a zooplankton community established with resting eggs from the bottom sediment of a lake. The fish strongly depressed the populations of most planktonic cladoceran and rotifer species but not the abundances of copepod nauplii or the periphytic rotifers Monostyla spp. and Lecane spp. Gut content analyses showed that the fish larvae strongly selected for the periphytic rotifer Lecane spp.

Experimental analysis of the predation impact of the larvae of pond smelt (Hypomesus transpacificus nipponensis) on zooplankton populations established in mesocosms

The predation impact of the larvae of pond smelt Hypomesus transpacificus nipponensis on a zooplankton community was studied using mesocosms. The fish significantly depressed the abundances of copepod nauplii and rotifers, especially Hexarthra mira. The vulnerabilities of these prey might be determined by their swimming behavior and population density, suggesting that larval fish selectively prey on zooplankton that have a high encounter rate with the predator. The larvae did not have a negative effect on the densities of cladocerans, but fish predation altered the cladoceran community structure from the dominance of B. longirostris to that of B. fatalis. This result suggests that larval fish predation is an important factor that shifts the species composition of Bosmina in some lakes, the shift occurring in the season when fish larvae are abundant. Our results have shown that predation by the larval fish would control not only the abundance, but also the community structure of the small-sized zooplankton prey.

Day and Night Vertical Distributions of Rotiferan and Crustacean Zooplankton in Lake Suwa

ABSTRACT We studied the vertical distribution of zooplankton in shallow Lake Suwa monthly during two summers. Polyarthra spp. and Eodiaptomus japonicus showed marked diel changes in their vertical distribution patterns. E. japonicus displayed a typical migration, avoiding the surface and maintaining a high abundance in deeper water during the day. Polyarthra spp. showed the reverse migration pattern, with high abundance near the surface during the day and uniform distribution during the night. This distribution pattern seems to be related to the distribution of an invertebrate predator, Thermocyclops sp., which migrated downward during the day.

Egg viability of the rotifer Brachionus urceolaris after ingestion by the predatory cladoceran Leptodora kindtii

Egg resistance against the digestive process of a predator is an effective strategy for zooplankton to compensate population loss due to predation. Parthenogenetic eggs of the rotiferan Brachionus urceolaris, which were ingested by the predatory cladoceran Leptodora kindtii, were expelled from the feeding basket of the predator without digestion. We found a negative correlation between the unconsumed ratio of eggs after ingestion and body length of the predator. As high as 75% of the unconsumed eggs successfully hatched and the hatch ratio was independent of body length of L. kindtii. Our results indicate that the rotifer has an effective strategy to maintain its population in the environment with abundant invertebrate predators.

Long-term zooplankton community records (1996–2017) for Lake Suwa (Japan)

The impact of eutrophication on aquatic ecosystems remains an important topic in aquatic ecology; however, recent successes in water quality restoration in highly eutrophicated water bodies present new research potential regarding re-oligotrophication. Successfully reducing nutrient loading from sewage treatment through restoration activities, induces large changes in phytoplankton composition and biomass, particularly replacement of cyanobacterial dominance. In Lake Suwa, a shallow eutrophic lake in central Japan, recovery has occurred due to water quality restoration efforts since the 1970s. The improvement of lake trophic state from hypertrophic to mesotrophic is accompanied by various changes, such as rapid decreases in biomass of phytoplankton, benthic invertebrates and planktivorous pond smelt, and increases in biomass of aquatic vegetation, mainly floating leaved plants. During re-oligotrophication, zooplankton are important because they are major secondary producers in lake ecosystems. In Lake Suwa, the Research and Education Center for Inland Water Environment, Shinshu University has collected bi-weekly zooplankton samples and analyzed species composition since 1996, when the lake was in a hypertrophic state with serious Microcystis blooms. Lake Suwa is one representative lake for re-oligotrophication in a shallow eutrophic system, and our zooplankton dataset can be used to understand the changes in ecosystem structure and function.

Changes in ecosystem structure associated with the restoration of water quality in the shallow eutrophic Lake Suwa, Japan

Deterioration ofwater quality in lakes due to inereased nutrient eoneentration (eutrophieation), whieh induees eyanobaeterial blooms (HoRNE & GoLDMAN 1994), is a major environmental problem. Many attempts have been made to lower the nutrient eoneentration by redueing extemal and interna! nutrient loadings; however, results have not been positive in many lakes, espeeially shallow lakes. The restoration of water quality has markedly proeeeded reeently in the shallow Lake Suwa, however, where eeosystem strueture has been ehanging with ehanges in water quality. Our researeh analyzes the ehange in eeosystem strueture and its meehanism. In this study, we summarize the reeent results of the projeet and diseuss the faetors inducing the eeosystem ehanges.

Impact of invertebrate predators on rotifer populations in lakes: field observation and experimental analysis

Invertebrate predation greatly affeets the strueture of freshwater zooplankton eommunities (KERFOOT 1977, HANAZATO & YASUNO 1989, PLAGMANN et al. 1997). The rotifer eommunity, a group of freshwater zooplankton, is exposed to high predation pressure by eopepods and the predatory rotifer Asplanchna (GILBERT & WILLIAMSON 1978, WtLLWvfSON 1984, 1987). The prey-predator interaetions between rotifers and predators have been analyzed in many studies. For example, GtLBERT ( !980) direetly observed the feeding behavior of Asplanchna girodi on some prey types and found that the predator showed se!eetive predation and easily ingested Synchaeta pectinata, Keratella coch!earis, and uneolonized Conochilus. LAIR (!990) eoneluded from field observation that A. priodonta exerted predation pressure o n the population dynamies of K. cochlearis, Polyarthra remata, and P dolichoptera, and that the eyclopoid eopepod Cyclops vicinus vicinus had an negative impaet on the latter two rotifer speeies. Most studies have mainly analyzed interactions between a single invertebrate predator and several rotifer prey in the laboratory, or the seasonal succession of predator and prey populations in one or a few !akes. However, laboratory studies ean not assess the population-level impaet on rotifers by predators, and field observations ean not evaluate the predation impaet clearly beeause dynamics o f rotifer populations are affected not only by predation, but also by many environmental faetors. In the present study, we tried to estimate the impact of invertebrate predators on rotifer populations by eomparing zooplankton eommunities of 30 lakes with different environmental eonditions in the same season, and in a mesocosm experiment in which the density o f cyelopoid eopepod was control!ed.