Bibliothek

Notizen: 1. Day (noon) and night (midnight) vertical distributions of zooplankton and phytoplankton in the water column (1.5 m) of a Vermont pond were determined on two consecutive days from 470 mL water samples taken at three depths (0.1, 0.5 and 1.0 m) at three sites. There was little variation across depths in temperature, dissolved oxygen concentration and phytoplankton. All individuals of each zooplankton species (a small copepod, Tropocyclops extensus and six rotifers) were counted.2. A three-way ANOVA on the zooplankton data showed no effect of date or time of day on the abundance of any species. Significant diel shifts in vertical distribution (depth × time-of-day interactions) were found for T. extensus (nauplii, as well as copepodites and adults) and Polyarthra remata, but not for Hexarthra mira, Keratella cochlearis, Anuraeopsis fissa, Ascomorpha ovalis and Plationus patulus. Tropocyclops extensus showed a pronounced, typical diel vertical migration, avoiding the surface and occurring most abundantly near the bottom during the day. Polyarthra remata showed an equally pronounced, reverse diel vertical migration, avoiding the bottom and being most abundant near the surface during the day.3. The diurnal descent of Tropocyclops is interpreted as an avoidance response to Buenoa macrotibialis, a notonectid which feeds on this copepod at the surface during the day but not at night. The diurnal ascent of Polyarthra is thought to be an avoidance response to Tropocyclops, which strongly suppresses this rotifer in field enclosures and laboratory vessels. Thus, these out-of-phase migrations may be coupled and represent a behavioural cascade initiated by Buenoa.4. At night, Tropocyclops and Polyarthra both were uniformly distributed across depths. This is believed to reflect the absence of appreciable depth-related variation in temperature, algal food resources (biovolume of cryptomonads and chrysophyte flagellates) and predation risk at this time.5. The five rotifer species that did not exhibit diel vertical migrations may be less susceptible to Tropocyclops predation than Polyarthra.

Notizen: 1. Interactions were observed between three morphotypes of the predatory rotifer Asplanchna silvestrii and six different prey (Brachionus plicatilis, B. rotundiformis, B. pterodinoides, B. satanicus, Hexarthra jenkinae and copepod nauplii) to understand the differences in feeding abilities among morphotypes that may have led to the evolution of this predator polymorphism. The outcome of predation events was affected significantly, both by predator morphotype and prey type. Predator morphotypes also interacted differently with different prey types.2. The two smaller morphotypes, the saccate and the cruciform, responded similarly to prey overall, except that the smallest morphotype (saccate) was unable to ingest the most mobile prey (nauplii) and less able to ingest relatively large prey (B. plicatilis). The largest morphotype, the campanulate, had the highest encounter rate with prey, but the lowest probability of attack after encounter, so that it consumed far fewer prey per feeding bout than did the smaller morphotypes. This may have been because campanulates prefer larger prey than used in this study.3. Highly mobile prey (H. jenkinae and copepod nauplii) were much less susceptible to predation than the less mobile Brachionus species. While evasiveness reduced attacks by saccates and cruciforms, campanulates did not have a significantly lower attack rate on H. jenkinae and copepod nauplii than on less evasive prey. Large body size moderately defended B. plicatilis against ingestion by saccates only. The short-spined B. satanicus was the only prey that was rejected after capture, resulting in lower ingestion probabilities for B. satanicus than other brachionid prey.

Notizen: 1. This synthesis examines 35 long-term (5–35 years, mean: 16 years) lake re-oligotrophication studies. It covers lakes ranging from shallow (mean depth 〈5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L−1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north-temperate lakes were most abundant.2. Reduction of external total phosphorus (TP) loading resulted in lower in-lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially.3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in-lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables.4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria.5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of 〈100–150 μg L−1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters.6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity.7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re-oligotrophication.

Notizen: Abstract Several species of the predatory rotifer Asplanchna exhibit dramatic diet-induced trimorphism. The three morphotypes differ greatly in body shape and size, attributes that should affect predation ability. It has been hypothesized that these morphotypes evolved to exploit different prey assemblages in different environments. Here I compare the predatory behavior of the campanulate morphotype (the largest) to that of the cruciform morphotype (the intermediate) using crustacean and conspecific prey. These prey are known to induce production of the greatest proportion of campanulates. I hypothesize that the campanulate is better able to exploit these relatively large prey than are cruciforms. The campanulates did have higher ingestion rates with conspecific prey, but the ingestion rates of the morphotypes were not different with the crustacean prey, due to the campanulate's relatively low probability of attacking the crustacean prey. The campanulate attack probability is higher with both conspecific and crustacean prey than has been previously reported for campanulate A. silvestrii with smaller rotifer prey. While the campanulate handles both relatively large prey with comparative ease, and is more likely to attack these prey than smaller rotifer prey, the campanulate morphotype seems most effective at cannibalism due to its high preference for congeneric prey.

Notizen: Abstract Several species of the predatory rotifer Asplanchna exhibit dramatic diet-induced trimorphism. The three morphotypes differ greatly in body shape and size, attributes that should affect predation ability. It has been hypothesized that these morphotypes evolved to exploit different prey assemblages in different environments. Here I compare the predatory behavior of the campanulate morphotype (the largest) to that of the cruciform morphotype (the intermediate) using crustacean and conspecific prey. These prey are known to induce production of the greatest proportion of campanulates. I hypothesize that the campanulate is better able to exploit these relatively large prey than are cruciforms. The campanulates did have higher ingestion rates with conspecific prey, but the ingestion rates of the morphotypes were not different with the crustacean prey, due to the campanulate's relatively low probability of attacking the crustacean prey. The campanulate attack probability is higher with both conspecific and crustacean prey than has been previously reported for campanulate A. silvestrii with smaller rotifer prey. While the campanulate handles both relatively large prey with comparative ease, and is more likely to attack these prey than smaller rotifer prey, the campanulate morphotype seems most effective at cannibalism due to its high preference for congeneric prey.