摘要:

I examined whether light availability mediated the extent to which herbivory by green frog tadpoles (Rana chmitans) affected lower trophic levels in a fishless pond in central Kentucky. An enclosure experiment manipulating tadpole presence/absence and light (shaded vs. unshaded) showed that tadpoles can significantly reduce benthic algal biomass in light levels that mimic springtime canopy conditions. Tadpoles, however, had little affect on algal species composition. Grazing did not indirectly affect nutrient (NO3and PO4) concentrations in experimental treatments, probably because ambient concentrations were beyond growth-limiting levels.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663584

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Walleye (Stizostedion vitreum vitreum) were introduced into a 4.3 ha pond near Yakima, Washington, April, 1993 to control an overabundant and stunted population of yellow perch (Perca flavescens). Upon examination of the stomach contents from 116 walleye, we found 43 had food in their stomachs. Of these, pumpkinseed sunfish (Lepomis gibbosus) represented 34% by weight and 48% by number of the total contents, while yellow perch represented only 29% by weight and 11 % by number. After a survey of available forage in the pond, we determined that the walleye were targeting pumpkinseed sunfish over yellow perch both in the fall of 1993 (P< 0.001) and the fall of 1994 (P< 0.025). Our results contrast with that of others suggesting that walleye do not feed heavily on centrarchids. The vulnerability of different centrachids to walleye predation can vary by species but our study suggests that walleye could be introduced to shallow waters to control some species of overabundant centrarchids, particularly pumpkinseed sunfish.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663585

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Current research has demonstrated that a guild of at least a dozen diatom genera dominates the autotrophic base of the systems in the Perkiomen and Skippack Creeks in Montgomery County, Pennsylvania. Previous research in the Skippack alone demonstrated that the common genera exhibited distinct periodicities and suggested that total diatom densities were similar from sampling dates centered around the autumnal and vernal equinoxes. We investigated the question of whether the diversity, richness, and equitability of the diatom guild were the same or different at the equinoxes. Collections at fourteen day intervals centered around the equinoxes of 1992–3 and 1994–5. Densities per date were determined by Whipple field counts, and analyses of diversity, richness, equitability, and community similarity were made using Simpson's Index of Diversity and Sorensen's Coefficient of Similarity. We found that the diatom communities are not the same at the equinoxes, including their total densities and dominant genera. Generic richness varied little between seasons, but the systems were more diverse at the vernal equinox because of changes in equitability.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663586

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

High abundance of filter-feeding lotic insects near lake outlets has been attributed to the high quality/quantity of seston available in this habitat. Accordingly, a decrease in food quality or quantity with increased distance from a lake outlet might be detectable as a decrease in insect size. We examined changes in larval size of the black flySimulium truncatumwithin the first 120 m of a stream draining a small lake from the Avalon Peninsula, Newfoundland. The observed decrease in larval size with increased distance from the outlet was consistent with a decreasing food hypothesis. Laboratory experiments also showed that larval size was dependent on food supply.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663587

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Ultraviolet (UV) radiation levels were manipulated in the littoral zone of a low-elevation, mid-latitude glacial lake. Benthic algal biomass accrual was significantly inhibited by ambient levels of W radiation early in colonization, but after 45 days of accumulation, the biomass of W-exposed and W-shielded algal communities did not differ significantly. Algal taxa differed in their sensitivity to UV radiation. Cell densities ofSynedra ulna, S. radians, Cyclotella comensis, and the Epitherniaceae were significantly reduced by ambient UV radiation.Achnanthidium minutissimumappeared to be stimulated by exposure to UV radiation. Differential responses of diatom taxa may have important implications for littoral zone food webs.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663588

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Based on studies conducted in 1987 and 1993, we have determined that the Asiatic clam,Corbicula fluminea, has moved upstream at least 1.2 km/year in a southeastern U.S. blackwater stream. Since these studies were done in a protected area (the Savannah River Site, South Carolina), our data suggest that this was a process unaided by humans. In addition, the upstream movement has been halted by a large, permanent wetland and two culverts. Because the Asiatic clam has not colonized headwater areas, where several wetlands exist, this precludes the aided movement ofC. flumineaby waterfowl. It is possible that fish are partially responsible for dispersingC. fluminea, at least for longer distances. We suggest that unassisted upstream movements may also be an important dispersal mechanism for this species.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663589

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The diurnal vertical distribution and drift of zooplankton were examined in a deep (2.6 m), second order, stream pool in northcentral Arkansas. Zooplankton densities were substantial (max. > 1500/L) with the majority of the organisms occurring near or at the bottom. A significant increase in the density of copepods and rotifers occurred at 1 m during afternoon and evening hours, respectively. No increase was observed for Cladocera. Significantly more zooplankton drifted into the pool at night than drifted out. Increased densities during the night at both upstream and downstream sites were significant for all zooplankton groups except copepods at the downstream site.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663590

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We measured morphometric and meristic parameters of gill rakers from the first gill arch of 36 adult lake herring (Coregonus artedi) from Lake Superior that ranged in length from 283–504 mm. These data, coupled with the mean of the smallest two body dimensions (length, width, or breadth) of various zooplankton prey, allowed us to calculate retention probabilities for zooplankton taxa that are common in Lake Superior. The mean of the smallest two body dimensions was positively correlated with body length for cladocerans and copepods. The large cladoceran,Daphnia g. mendotae, is estimated to be retained at a greater probability (74%) than smaller cladocerans (18%-38%). The same is true for the large copepod,Limnocalanus macrurus(60%), when compared to smaller copepods (6–38%). Copepods have a lower probability of being retained than cladocerans of similar length. Lake herring gill rakers and total filtering area are also positively correlated with fish total length. These data provide further evidence that lake herring are primarily planktivores in Lake Superior, and our data show that lake herring can retain a broad range of prey sizes.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663591

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The freshwater limpet,Ferrisla rivularis, was found in several watersheds in south-central and south-western Pennsylvania. Shell size was not significantly correlated with stream pH, calcium, iron, or manganese concentrations. Limpets tolerated stream iron concentrations up to 0.36 mg/l and manganese concentrations up to 0.25 mg/l. The shells of these snails were quickly and heavily colonized by adherent or embedded diatoms, and shell size was significantly positively correlated with diatom coverage (P=0.050). Diatom coverage was significantly positively correlated with stream phosphate concentration (P=0.002).

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663592

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The methodology used for microscopically examining and enumerating phytoplankton has a major influence on the results obtained. This study describes how counting or not counting picoplankton (cells less than 2 μm) makes a large difference in terms of total abundance and taxonomic composition estimates. We suggest that the type of methodology including the smallest level of detection attempted in terms of size (e.g. macroplankton, net plankton, nannoplankton or picoplankton), should be reported for all phytoplankton studies where organisms are counted. It is also important to restrict data comparisons to studies where the same level of detection is used.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663593

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor