Many studies have reported feeding habits of the introduced largemouth bass, Micropterus salmoides, associating their presence with ecological impacts on native prey communities. However, few studies have been conducted on the dietary changes of the bass after exploitation of the native communities. In this study, we investigated the feeding habits of an introduced largemouth bass population in Lake Shirakaba, in which biomanipulation substantially depressed a planktivorous fish population and, in-turn, increased the meso-zooplankton density. Following the manipulation, the bass population shifted its diet from the planktivorous fish to the meso-zooplankton. Although the ontogenetic niche shift from planktivory to piscivory has been frequently reported in this species, few studies have reported that largemouth bass adults show strong reliance on zooplankton prey. A possible mechanism for dietary changes in this population is a flexible response to food availability in changing prey communities.
In oligotrophic lakes, littoral benthic communities are diverse with due to multiple supply routes of production. In the present study, the spatial and temporal patterns of benthic assemblages and their potential food resources were investigated for three habitat types (cobble, pebble and sand) in the littoral zone of a large oligotrophic lake, Lake Shikotsu of northern Japan, from June 2006 to June 2007. A total of 22,112 observations representing 47 taxa were recorded. Spatial and temporal patterns of taxa richness, number of individuals and biomass of benthic assemblages revealed high variation among habitat types and across sampling periods. On a spatial scale, taxa richness, number of individuals and biomass of benthic assemblages in the cobble habitat showed the highest values among the three habitat types. Three potential food resources (chlorophyll a of algae, coarse particulate organic matter and fine particulate organic matter) also indicated high variation among sampling sites and sampling periods, with the most abundant resources found in the cobble habitat. Among the four functional feeding groups of benthic assemblages examined, collector-gatherers (49–81%) represented the largest number of individuals (herbivores, 20–35%, predators, 11–15%, and shredders, 1–5%). Dominant taxa collected in this study consisted of two mayflies (Paraleptophlebia sp. and Caenis sp.) and four midges (Orthocladiinae, Tanypodinae, Diamesinae and Chironominae). Five out of the six dominant taxa belonged to collector-gatherers, with the exception of Tanypodinae. Despite such high value of collector-gatherers in benthic assemblages, multiple regression analysis indicated that chlorophyll a and coarse particulate organic matter were better predictor elements in potential food resources for benthic assemblages than fine particulate organic matters. Our results suggest that spatial and temporal patterns of benthic assemblages in littoral zones of oligotrophic lakes exhibit high variation in abundance, reflecting the complex relationship with substrate type, canopy opening and resource abundance in each habitat.
Occasional summer cyanobacteria blooms (Aoko) occur in Lake Shinji which is located in the eastern Shimane Prefecture. To clarify the cause of cyanobacteria bloom occurrence, we conducted discriminant analysis between cyanobacteria bloom occurrence, Cl- concentration, water temperature, NO3-N concentration, PO4-P concentration, duration of sunshine, and the flow rate from the Hii River from 1984 to 2012. We found that three of the most relevant variables are Cl- concentration during the index month, Cl- concentration two months prior to the index month, and water temperature one month prior to the index month. This discrimination is 89.7% correct. In the present study, “index month” indicates the month in which cyanobacteria blooms occur, and if not, indicates the month of August. Pearson's correlation coefficient between Cl- concentration during the index month and Cl- concentration one month prior to the index month is high (0.78). Next, we conducted discriminant analysis using Cl- concentration (one month prior), Cl- concentration (two months prior), and water temperature (one month prior) as variables. This discrimination is 86.2% correct. Our study determined that the analysis of Cl- concentration one month prior to the index month yields similar results to the analysis of Cl- concentration during the index month. Based on our study results, we confirmed that we can predict cyanobacteria bloom occurrence one month prior to bloom occurrence.
To clarify the temperature dependence of dissolved organic carbon (DOC) concentration, we sampled water once per week at the center of Lake Shinji from July 1, 2010 to June 27, 2011. Maximum, minimum, average, and median total organic carbon (TOC) concentrations were 4.7, 1.9, 2.8, and 2.6 mg L-1, respectively. DOC concentrations of these concentrations were 2.7, 1.3, 1.9, and 1.8 mg L-1, respectively. By dividing the DOC by the TOC of these sample we found ratios of 0.86, 0.47, 0.70, and 0.72, respectively. A positive correlation was found between water temperature and DOC, and there was a time lag between temperature increase/ decrease and DOC increase/ decrease. The correlation coefficient for the simple regression between temperature and DOC improved when we delayed temperature data. The highest coefficient of correlation was gained when we added 8.6 weeks (60 days) to temperature data.
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