Resource switching in fish following a major food web disruption.

Dreissenid mussels (Dreissena polymorpha and D. bugensis) have re-engineered Great Lakes ecosystems since their introduction in the late 1980s. Dreissenids can have major indirect impacts on profundal habitats by redirecting nutrients and energy away from pelagic production (which supplies profundal production) and depositing nutrients and energy in the nearshore zones that they occupy. However, strong empirical evidence for the effects of this redirection of resources on fish populations is currently lacking. Here, we report significant shifts in isotopic signatures, depth distribution and diets of a coldwater profundal fish population that are all consistent with a greater reliance on nearshore resources after the establishment of dreissenid mussels in South Bay, Lake Huron. Isotopic signatures of scales collected from 5-year-old lake whitefish (Coregonus clupeaformis) demonstrated remarkable stability over the 50-year period prior to the establishment of dreissenids (1947-1997) and a sudden and significant change in isotopic signatures (3 per thousand enrichment in delta13C and 1 per thousand depletion in delta(15)N) after their establishment (2001-2005). These dramatic shifts in isotopic signatures were accompanied by a coincident shift in the mean depth of capture of lake whitefish towards the nearshore. A comparison of previously unpublished pre-invasion diets of lake whitefish from South Bay with contemporary diets collected between 2002 and 2005 also indicate a greater reliance on nearshore prey after the invasion of dreissenid mussels. This study is the first to report changes in the carbon source available to lake whitefish associated with restructured benthic communities after the appearance of dreissenid mussels. Further, this study contributes to a growing body of work that demonstrates the ecological insights that can be gained through isotopic analysis of archived fish bony tissues in ecosystems that have experienced significant levels of disturbance.

Limitations of fluoridation effectiveness studies: Lessons from Alberta, Canada.

A paper published in this journal, "Measuring the short-term impact of fluoridation cessation on dental caries in Grade 2 children using tooth surface indices," by McLaren et al had shortcomings in study design and interpretation of results, and did not include important pertinent data. Its pre-post cross-sectional design relied on comparison of decay rates in two cities: Calgary, which ceased fluoridation, and Edmonton, which maintained fluoridation. Dental health surveys conducted in both cities about 6.5 years prior to fluoridation cessation in Calgary provided the baseline. They were compared to decay rates determined about 2.5 years after cessation in a second set of surveys in both cities. A key shortcoming was the failure to use data from a Calgary dental health survey conducted about 1.5 years prior to cessation. When this third data set is considered, the rate of increase of decay in Calgary is found to be the same before and after cessation of fluoridation, thus contradicting the main conclusion of the paper that cessation was associated with an adverse effect on oral health. Furthermore, the study design is vulnerable to confounding by caries risk factors other than fluoridation: The two cities differed substantially in baseline decay rates, other health indicators, and demographic characteristics associated with caries risk, and these risk factors were not shown to shift in parallel in Edmonton and Calgary through time. An additional weakness was low participation rates in the dental surveys and lack of analysis to check whether this may have resulted in selection biases. Owing to these weaknesses, the study has limited ability to assess whether fluoridation cessation caused an increase in decay. The study's findings, when considered with the additional information from the third Calgary survey, more strongly support the conclusion that cessation of fluoridation had no effect on decay rate. Consideration of the limitations of this study can stimulate improvement in the quality of future fluoridation effectiveness studies.

Predator-induced behavioral defense and its ecological consequences for two calanoid copepods.

We used an automated technique for the observation and quantification of zooplankton swimming behavior to study the behavioral responses of two congeneric, herbivorous, freshwater copepod prey to a copepod predator (Limnocalanus macrurus). One prey, Diaptomus sicilis, often co-occurs with Limnocalanus, while previous studies indicated that the zoogeographic distribution of the second prey, Diaptomus oregonensis, was independent of the predator. We found that in the presence of Limnocalanus, D. sicilis swims more slowly and with less hopping and jumping than D. oregonensis. Diaptomus sicilis is also attacked and consumed by the predator Limnocalanus macrurus less frequently than D. oregonensis. We suggest that the faster, "noisier" swimming of D. oregonensis increases its vulnerability to Limnocalanus. The behavioral defenses to both prey are induced by the presence of the predator, and may represent two different anti-predator strategies, "crypsis" and "avoidance" for D. sicilis and D. oregonensis respectively. In a zoogeographical analysis D. oregonensis occurs at densities below D. sicilis in lakes where Limnocalanus is at elevated abundances, while in "low-predator" lakes the opposite is true. This distribution pattern supports our experimental results, and suggests that D. sicilis is adapted to survive with Limnocalanus, while D. oregonensis is not.

Use of enclosures to detect the contribution of particular zooplankton to growth of young-of-the-year yellow perch (Perca flavescens Mitchell).

Densities of the cladoceran, Holopedium gibberum, were manipulated in 18 enclosures containing juvenile (age 0+) yellow perch (Perca flavescens) and mean-lake densities of other zooplankton. In enclosures, where nearlake densities of all zooplankton species including Holopedium were maintained, young-of-the-year perch grew significantly heavier and longer than in experimental enclosures where Holopedium was excluded. Holopedium comprised between 15-45% of the diet (wet weight) of perch in the first 2 weeks of July in the control treatment (Holopedium at or near ambient lake densities) and only 3-7% of total biomass ingested in the experimental treatment (Holopedium density selectively reduced). Predation on Holopedium decreased dramatically after the 2nd week of July in the control treatment after which Chaoborus, chironomids, and Sida became dominant prey items (by weight) of juvenile perch. These findings suggest that growth and survivorship of age 0+ perch in Precambrian Shield lakes may be coupled to Holopedium abundance. Thus, utilization of Holopedium by young-of-the-year yellow perch may affect recruitment of this species since overwintering survivorship, range of accessible prey sizes or species, and vulnerability of juvenile perch to predation by larger fish depend on body size, which is reduced when Holopedium is excluded from the diet.