Changes in water chemistry can disable plankton prey defenses

The effectiveness of antipredator defenses is greatly influenced by the environment in which an organism lives. In aquatic ecosystems, the chemical composition of the water itself may play an important role in the outcome of predator–prey interactions by altering the ability of prey to detect predators or to implement defensive responses once the predator’s presence is perceived. Here, we demonstrate that low calcium concentrations (<1.5 mg/L) that are found in many softwater lakes and ponds disable the ability of the water flea, Daphnia pulex to respond effectively to its predator, larvae of the phantom midge, Chaoborus americanus. This low-calcium environment prevents development of the prey’s normal array of induced defenses, which include an increase in body size, formation of neck spines, and strengthening of the carapace. We estimate that this inability to access these otherwise effective defenses results in a 50–186% increase in the vulnerability of the smaller juvenile instars of Daphnia, the stages most susceptible to Chaoborus predation. Such a change likely contributes to the observed lack of success of daphniids in most low-calcium freshwater environments, and will speed the loss of these important zooplankton in lakes where calcium levels are in decline.

The distribution of four Chaoborus species (Diptera: Chaoboridae) along an elevation gradient in Canadian Rocky Mountain lakes

We tested the hypothesis that the distribution of four species of Chaoborus is limited by water temperature in Rocky Mountain lakes. Midsummer surface water temperature (MSSWT) of Rocky Mountain lakes varied between 25 and 5 °C along an elevation gradient spanning 600–2400 m above sea level. Chaoborus (subgenus Chaoborus) americanus and C. (C.) flavicans were collected in lakes with MSSWT ≥ 16 °C, generally corresponding to lakes at elevations lower than 1600 m above sea level. Chaoborus (Sayomyia) punctipennis was only collected in warm lakes (MSSWT ≥ 21 °C). Species of the subgenus Schadonophasma (C. trivittatus and possibly C. cooki) were not commonly collected, but preliminary data suggest that they may be more tolerant of low water temperatures than the other species. On a qualitative basis, the distribution of these chaoborids in the Rockies is similar to their latitudinal distribution. However, MSSWT as a valid predictor of Chaoborus species distribution with latitude remains to be tested. The gradient in lake temperature found in mountainous environments appears to be a useful gauge for obtaining information about the distribution of invertebrates relative to temperature.

Historical Status of Fish Populations in Canadian Rocky Mountain Lakes Inferred from Subfossil Chaoborus (Diptera: Chaoboridae) Mandibles

We used subfossil Chaoborus mandibles preserved in sediments of Cabin, Caledonia, and Celestine lakes, Alberta, to determine whether fish were present in the lakes (which now contain rainbow trout (Oncorhynchus mykiss) and lake chub (Couesius plumbeus)) prior to the first fish stockings early this century. We first tested whether Chaoborus were good indicators of the presence of fish in montane lakes in a survey of 43 lakes in jasper and Banff national parks. Chaoborus americanus was the only species inhabiting fishless lakes and was also found in three lakes with low fish density (co-occurring with C. flavicans in two of the three lakes). Other lakes containing fish had either C. flavicans or no Chaoborus species. Subfossils revealed that C. americanus was the predominant species prior to this century in Cabin and Celestine lakes, suggesting that these lakes were originally fishless; C. americanus (and C. trivittatus in Cabin Lake) were eliminated early this century, contemporarily with the first recorded fish stockings. Chaoborus flavicans was the only species recovered from Caledonia Lake, suggesting that the lake had fish prior to this century; while it is not possible to identify the species originally present, historical evidence suggests that rainbow trout is native to Caledonia Lake.

Differential Predation by Chaoborus americanus on Males and Females of Two Species of Diaptomus

Feeding experiments were performed to determine relative feeding rates of fourth-instar Chaoborus americanus larvae (Diptera) on both sexes of two calanoid copepod species, Diaptomus leptopus and D. minutus. Feeding rates, using a single prey type, showed that predation rates by Chaoborus on female D. minutus were highest, followed by male D. minutus, male D. leptopus, and female D. leptopus, respectively, when exposed to Chaoborus individually. Selectivity experiments with the four prey types made available simultaneously confirmed this pattern. Prey sizes and swimming speeds were determined, and both strike efficiency and handling time of Chaoborus were measured. Data suggested that selection between prey species was determined primarily by prey size whereas selection between sexes was determined primarily by differential swimming speed.