bluuebunny/crossref_metadata_2025 huggingface dataset:
The development of recruitment fisheries oceanography in the United States
Authors: KENDALL ARTHUR W. , DUKER GARY J.
Published: March 2003

Recruitment fisheries oceanography studies the impact of the environment on the annual production of young to fished populations (finfish as well as invertebrates). Interannual variation in recruitment is the most important source of biological variability facing fisheries managers. Because most variation in recruitment occurs during early, mainly planktonic stages, recruitment fisheries oceanography usually integrates studies of plankton and physical oceanography. The concepts upon which these studies rest were first expressed in the late 1800s by Spencer Fullerton Baird, the first Commissioner of the US Commission of Fish and Fisheries. These concepts appear to have been independently developed by Johan Hjort and others in northern Europe in the early 1900s, and brought back to the United States through contacts between Hjort and Henry Bryant Bigelow, who passed the ideas to his students at Harvard University, including Lionel Albert Walford and Oscar Elton Sette. Although both Walford and Sette did their initial work in recruitment fisheries oceanography off the US east coast, as federal fisheries scientists, they were sent to California in response to the decline of the sardine fishery, where they incorporated the ideas of Hjort into the programme that has become the California Cooperative Oceanic Fisheries Investigations (CalCOFI). The original plan for CalCOFI research was to provide a test of Hjort's ideas. Scientists working with CalCOFI implemented this plan and conducted subsequent research that had its roots in the ideas expressed by Baird. This research was in marked contrast to the fishery‐yield orientation of most fisheries research that was being conducted at the time on the west coast of North America, under the dominating influence of William Francis Thompson. In recent years, federal fisheries programmes have investigated recruitment processes of a number of other fish stocks, and considerable effort has been expended toward refining the conceptual framework beyond the hypotheses of Hjort. This paper expands on this history, making note of scientists who were particularly important in the evolution of this discipline. We conclude that although recruitment fisheries oceanography has become a well‐established field of study, and many technological advances have been made, the recruitment process is still not well understood and fluctuations in year‐class abundance remain a major source of uncertainty in managing marine fisheries.
Population dynamics of Calanus chilensis in the Chilean Eastern Boundary Humboldt Current
Authors: ESCRIBANO RUBEN
Published: March 2003

A study of the population dynamics of Calanus chilensis was carried out in the coastal area of the Mejillones Peninsula (23°S), which is an upwelling site in the Eastern Boundary Humboldt Current. Information was derived from time‐series sampling (15 day interval) at three stations, and from two broad‐scale cruises: under active upwelling, and under 1997/1998 El Niño conditions. Calanus chilensis showed continuous reproduction throughout the year. In the study area, upwelling takes place intermittently year‐round, suggesting that Calanus production is not limited by food. Advection played an important role in structuring the spatial distribution of Calanus, both during upwelling and under El Niño conditions. Population losses from upwelling and offshore transport may be compensated by rapid turnover rates of cohorts, growing at temperature‐dependent rates and hence allowing population recovery, even in abnormally warm waters during El Niño conditions.
Seasonal modulation of microbially mediated carbon fluxes in the northern Adriatic Sea — a model study
Authors: VICHI M. , ZAVATARELLI M. , PINARDI N.
Published: March 2003

A model study of the seasonal changes in the food web structure of the northern Adriatic Sea is presented. A 1–D ecosystem coupled model (POM/ERSEM) forced with climatological functions has been applied at two different station locations in the northern Adriatic Sea, one under the direct influence of the Po river inflow and one representing a more open sea situation. The ability of the model to reproduce the main physical and biogeochemical patterns allows us to study the underlying carbon fluxes, which determine these features. Analysis of the seasonal carbon‐transfer pathways among the main trophic levels shows a possible seasonal modulation at both stations, but a different food web development. It seems that the prevailing path could be related to the seasonal variability of the water column (nutrient input and/or vertical stratification). Moreover, results from this numerical experiment suggest that the northern Adriatic shallow coastal area has a predominantly multivorous food web and has significant microbial recycling, even if this latter pattern is not so evident as in open sea areas.
Annual weather variability and its influence on the population dynamics of Calanus finmarchicus
Authors: RADACH GÜNTHER , CARLOTTI FRANCOIS , SPANGENBERG ANDREAS
Published: March 2003

The reaction of the population of Calanus finmarchicus to relatively extreme annual cycles of weather in the North Sea was investigated by one‐dimensional model simulations. A population dynamics model for C. finmarchicus was coupled with a physical and a biological upper‐layer model for phosphate, phytoplankton and detritus to simulate the development of the successive stages of C. finmarchicus. Observed annual weather cycles were used to drive the physical water column model, the results of which were then input to the plankton model. The simulations yielded the temporal development of all stages of C. finmarchicus over an annual cycle in terms of numbers and weights. Compared with the results reported for 1984 by Carlotti and Radach (1996; Limnol. Oceanogr. 41: 522–539), it appears that the temporal range for the occurrence of the peak concentration of C. finmarchicus may be 2 weeks in the northern North Sea. The simulated variability is in accordance with observations. Years with more continuous primary production such as the year 1984, which was cloudy and windy during the summer, may then provide the optimum conditions with respect to producing an abundance of zooplankton during the following year.
The multinomial logit model: a new tool for exploring Continuous Plankton Recorder data
Authors: Beare D. , McKenzie E.
Published: March 2003

The use of Generalized Additive Models in the analysis of Continuous Plankton Recorder (CPR) distribution and abundance data for Calanus finmarchicus and C. helgolandicus is demonstrated. Strong emphasis is placed on the influence of long‐term trend, seasonality and location. The preponderance of zeros in the data, as well as the particular problems associated with the CPR recording protocol, are adequately handled by the models. Three models are described. In the first two, data within arbitrary spatial compartments are modelled as smooth functions of long‐term trend and seasonality, assuming a spatial homogeneity within the area. In the third model, the spatio‐temporal distributions of abundances over the North Sea in specific months each year are modelled as a three‐dimensional smooth function of long‐term trend and location (latitude and longitude). The summaries produced by the models reveal dynamic seasonal, spatial and long‐term patterns of abundance, many of which have not been revealed by the previous analytical techniques employed.
Tidal and subtidal fluctuations in temperature, salinity and pressure for the winter 1996 larval ingress experiment—Beaufort Inlet, NC
Authors: Blanton  , Amft  , Luettich Jr  , Hench  , Churchill
Published: March 2003

A multidisciplinary field experiment was conducted to compare water properties, larval abundances, and transport and retention processes at Beaufort Inlet and two channels leading to the estuarine nursery grounds. Temperature, salinity and subsurface pressure were monitored in situ for a six‐week period during March and April 1996 in each channel. Intensive sampling was performed during two neap‐tide periods when water mass conditions in the estuary were significantly different. Currents were stronger in the eastern channel during both experiments. Ebb currents were stronger than flood in both channels. Decreasing subtidal sea level appeared to account for the stronger ebb currents. Subtidal sea level in the inlet responded optimally to north–south (along the inlet axis) wind stress and along a line 15° clockwise of north–south. This direction closely parallels the channel axis of Core Sound and may provide an efficient conduit to carry large volumes of low‐salinity Pamlico Sound water into the estuarine complex when winds blow south in this sector. The tidal stream in Beaufort Inlet sets up strong cross‐inlet gradients by the advection on the east side of higher salinity shelf water and advection on the west side of Beaufort Inlet plume water. The axial fronts produced by differential advection of these two water masses might play some role in redistributing larvae present in one tidal stream to another.
Covariation in climate, zooplankton biomass and mackerel recruitment in the southern Gulf of St Lawrence
Authors: RUNGE  , CASTONGUAY  , LAFONTAINE Y. DE , RINGUETTE  , BEAULIEU
Published: March 2003

Between 1982 and 1991, an annual survey of stage I egg production of Atlantic mackerel (Scomber scombrus) was conducted in June/early July in the southern Gulf of St Lawrence. We investigated the relationship between interannual variability in biomass of zooplankton, determined from the archived survey plankton samples, and mackerel recruitment, estimated from the proportion of three‐year‐olds in the catch of the commercial fishery. Zooplankton biomass varied by a factor of 2.5, primarily owing to fluctuations in the >1000 μm size fraction. The index of mackerel recruitment fluctuated by a factor of ≈20 and was positively related (linear regression: P < 0.05; n = 10) to variations in the zooplankton biomass. Both mackerel recruitment and zooplankton biomass were negatively related (linear regression: P < 0.05) to RIVSUM, a measure of freshwater discharge from the St Lawrence River system and an index of variability in the region's climate. Three hypotheses are put forward to explain these observations: (1) there is a strong link between interannual variation in abundance of copepod females, which produce prey for mackerel larvae, and larval survival; the exceptional recruitment and subsequent year class resulted from an exceptional production of Calanus finmarchicus nauplii; (2) years of high zooplankton biomass provide better feeding conditions and consequently higher survival of mackerel juveniles; and (3) mackerel recruitment and zooplankton biomass are independently under the control of an underlying physical process, without strong trophic linkage. The first hypothesis is supported by a study of copepod species composition and female abundance conducted for four of the survey years. At the present time, none of these hypotheses can be ruled out.
Cod and haddock spawning on Georges Bank in relation to water residence times
Authors: Page F. H. , Sinclair M. , Naimie C. E. , Loder J. W. , Losier R. J. , Berrien P. L. , Lough R. G.
Published: March 2003

The seasonal and geographical patterns of spawning for Georges Bank cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) are estimated using composite distributions of stage I eggs derived from the Marine Resources Monitoring, Assessment and Prediction (MARMAP) data set (1977–1987). The inferred mean spawning locations are compared with patterns in particle residence times derived from a three‐dimensional prognostic circulation model that estimates the tidal and seasonal mean circulation. The comparisons indicate that cod and haddock spawning occurs at times and locations characterized by model residence times in excess of 35 d. The results are discussed in the context of fish population regulation theory, particularly the member/vagrant hypothesis and the concept of abundance‐dependent vagrancy.
Anomalous transport of walleye pollock larvae linked to ocean and atmospheric patterns in May 1996
Authors: Bailey Kevin M. , Bond Nicholas A. , Stabeno Phyllis J.
Published: March 2003

Larvae from a large aggregation of walleye pollock spawning in early spring in Shelikof Strait, Gulf of Alaska, are normally transported to the south‐west in the vigorous Alaska Coastal Current. In the spring of 1996, anomalous winds resulted in unusually weak transport in the Shelikof Strait sea valley. The main aggregation of larval pollock in the Shelikof region was surveyed four times in 1996 over a period of about 40 days, including finer‐scale sampling of the leading south‐western edge of the larval distribution. The south‐western edge of the larval distribution showed weak transport up the sea valley for a period of about 10 days, corresponding to the observations of currents, after which many larvae were transported over the shelf region to the west. These observations are unique in over 15 years of monitoring larval transport patterns and demonstrate how anomalous weather, and hence current patterns, influence variability in larval transport.
Modelling the transport of lobster (Homarus americanus) larvae and postlarvae in the Gulf of Maine
Authors: Incze Lewis S. , Naimie Christopher E.
Published: March 2003

We used a coupled physical–biological model to examine potential distances between hatching and settlement locations for lobsters in the Gulf of Maine. The physical model is based on a finite‐element mesh and climatological averages of the seasonally evolving temperature and density fields. Larval trajectories from coastal and offshore hatching sites (21–224 m deep) were calculated for early, middle and late‐season hatching by coupling temperature‐dependent development rates and depth (the biological model) to the circulation. Model results showed large spatial differences in larval development times (from 18 to 38 days) and distances transported (19–280 km) for the early hatch. Development time and transport decreased markedly by mid‐season at most sites, but strong spatial differences persisted. The eastern Maine coast appears to experience stronger removal and less resupply of larvae than other regions, consistent with observed lower recruitment. Inverse solutions of the model for larvae arriving in mid‐coastal Maine indicate that they originate from a broad section of the eastern coast ‘upstream’, with those nearest the shoreline generally travelling the shortest distances. The postlarval stage is neustonic (living near the surface), and a simple inverse model demonstrates that a diurnal coastal sea breeze can contribute substantially to inshore movement during this final planktonic stage. Thus, offshore reproduction may be linked to inshore recruitment.
Mid‐latitude wind stress: the energy source for climatic shifts in the North Pacific Ocean
Authors: Parrish  , Schwing  , Mendelssohn
Published: March 2003

Analyses of atmospheric observations in the North Pacific demonstrate extensive decadal‐scale variations in the mid‐latitude winter surface wind stress. In the decade after 1976 winter, eastward wind stress doubled over a broad area in the central North Pacific and the winter zero wind stress curl line was displaced about 6° southward. This resulted in increased southward Ekman transport, increased oceanic upwelling, and increased turbulent mixing as well as a southward expansion of the area of surface divergence. All these factors contributed to a decadal winter cold anomaly along the subtropical side of the North Pacific Current. In summer the cold anomaly extended eastward, almost reaching the coast of Oregon. The increased gradient in wind stress curl and southward displacement of the zero curl line also resulted in an increase in total North Pacific Current transport, primarily on the Equator side of this Current. Thus, surface water entering the California Current was of more subtropical origin in the post‐1976 decade. Southward (upwelling favourable) wind stress and sea surface temperature (SST) in the area off San Francisco exhibit at least three different types of decadal departures from mean conditions. In association with the 1976 climatic shift, marine fishery production in the Oyashio, California and Alaska Currents altered dramatically, suggesting that these natural environmental variations significantly alter the long‐term yields of major North Pacific fisheries.
Pacific herring,Clupea pallasi, recruitment in the Bering Sea and north‐east Pacific Ocean, II: relationships to environmental variables and implications for forecasting
Authors: Williams Erik H. , Quinn II Terrance J.
Published: March 2003

Previous studies have shown that Pacific herring populations in the Bering Sea and north‐east Pacific Ocean can be grouped based on similar recruitment time series. The scale of these groups suggests large‐scale influence on recruitment fluctuations from the environment. Recruitment time series from 14 populations were analysed to determine links to various environmental variables and to develop recruitment forecasting models using a Ricker‐type environmentally dependent spawner–recruit model. The environmental variables used for this investigation included monthly time series of the following: southern oscillation index, North Pacific pressure index, sea surface temperatures, air temperatures, coastal upwelling indices, Bering Sea wind, Bering Sea ice cover, and Bering Sea bottom temperatures. Exploratory correlation analysis was used for focusing the time period examined for each environmental variable. Candidate models for forecasting herring recruitment were selected by the ordinary and recent cross‐validation prediction errors. Results indicated that forecasting models using air and sea surface temperature data lagged to the year of spawning generally produced the best forecasting models. Multiple environmental variables showed marked improvements in prediction over single‐environmental‐variable models.
Mass mortality of short‐tailed shearwaters in the south‐eastern Bering Sea during summer 1997
Authors: Baduini C. L. , Hyrenbach K. D. , Coyle K. O. , Pinchuk A. , Mendenhall V. , Hunt G. L.
Published: March 2003

During summer 1997, hundreds of thousands of emaciated short‐tailed shearwaters (Puffinus tenuirostris) died in the south‐eastern Bering Sea. Using strip transect methodology, we documented the distribution and abundance of short‐tailed shearwaters during cruises conducted prior to, during, and after the die‐off, as well as the distributions and abundances of floating carcasses. The distributions and abundances of short‐tailed shearwaters in 1997 were similar to those found during the 1970s and early 1980s. In August–September 1997, we observed 163 floating shearwater carcasses, most of which were between St Paul Island and Nunivak Island. We estimated ≈ 190 000 carcasses were afloat in the study area, about 11% of the surveyed population. Between spring (June) and autumn (August/September), mean net body mass of shearwaters decreased by 19%, mean pectoral muscle mass decreased by 14%, and mean percentage body lipid content decreased by 46%, from 15.6% in spring to 8.4% in autumn. Compared with spring, short‐tailed shearwater diets broadened in autumn 1997, to include, in addition to adult euphausiids Thysanoessa raschii, juveniles of T. inermis, T. raschii and T. spinifera, crab megalops, fish and squid. We discuss how the ecosystem anomalies in the south‐eastern Bering Sea during spring and summer 1997 relate to the mortality event and suggest possible implications of long‐term climate change for populations of apex predators in the south‐eastern Bering Sea.
Temperature conditions in the Argentine chub mackerel (Scomber japonicus) fishing ground: implications for fishery management
Authors: Perrotta Ricardo G. , Viñas María D. , Hernandez Daniel R. , Tringali Leonardo
Published: March 2003

We describe the seasonal migrations of Argentine chub mackerel (Scomber japonicus) into the fishing ground in relation to sea surface temperature conditions and analyse the temperature evolution during the fishing season on the basis of 1955–97 time‐series. The upper temperature limit for the presence of schools was around 19°C. At higher values, most schools leave coastal waters (< 50 m) for the shelf. An inverse relationship was observed between catches per unit of effort (CPUE) of the inshore fishery and sea surface temperature. The highest standardized CPUE values corresponded to temperatures lower than 19°C. Above this temperature, the probability of obtaining a standardized CPUE higher than 1 was extremely low, and the standard deviations were minimal. The economic benefit from catches was also analysed through the different months of the fishing season. It was found that the mean benefit corresponding to temperatures lower than 19°C exceeded approximately 15 times that obtained at higher temperatures. A simple fishing strategy was proposed to the commercial fleet in order to improve the economic benefit of the catches. The migratory pattern followed by mackerel during the spawning period was also analysed in relationship to other environmental factors like food availability. Zooplankton composition, size structure and abundance in coastal and shelf waters are described, and their implications for feeding ecology of larvae and adults of the species are discussed.
Improvement in school‐size estimates of pelagic fish using information from sector‐scanning sonar
Authors: Hamano Akira , Nakamura Takeshi , Maeda Hiroshi
Published: March 2003

School‐size estimates based on information from a sector‐scanning sonar were compared with those based on information from an echo sounder. Three pairs of size estimates of anchovy schools in the coastal waters, off Fukuoka Prefecture were shown as examples. They included the possibility of improving the size estimates by using information from sector‐scanning sonar to consider the size in the transverse direction. In these examples, the thickness and extent of the school varied in the transverse direction. A large difference was found between the estimates from the sounder information and those from the sector‐scanning sonar information, although whether or not the estimated size decreased differed according to each case. Also, whether the thickness information or the extent information is influential differed according to each case. The size information from the sector‐scanning sonar in the transverse direction caused a change in the estimated size from 66 to 16 (×103 m3, mainly by different extent), 305 to 146 (×103 m3, thickness increased but extent decreased), and 819 to 746 (×103 m3, three seemingly separated schools fused into one).
Experimental measurements of zebra mussel (Dreissena polymorpha) impacts on phytoplankton community composition
Authors: Bastviken David T. E. , Caraco Nina F. , Cole Jonathan J.
Published: March 2003

1. To investigate direct effects of zebra mussel (Dreissena polymorpha) feeding activities on phytoplankton community composition, short‐term microcosm experiments were performed in natural water with complex phytoplankton communities. Both gross effects (without resuspension of mussel excretions) and net effects (with resuspension) were studied.

2. Gross clearance rates were not selective; essentially all taxa were removed at similar rates ranging from 24 to 63 mL mussel–1 h–1. Net clearance rates were highly selective; different plankton taxa were removed at very different rates, ranging from 12 to 83% of the gross rates, leading to consistent changes in the phytoplankton community composition. Thus, although zebra mussels can cause most phytoplankton to decline, there is considerable variation among taxa in either pre‐digestive selection or post‐digestive survival.

3. The direct, short‐term effects of zebra mussels on phytoplankton community composition are consistent with some of the major changes observed in the Hudson River since establishment of zebra mussels.

4. We show, with simple calculations, how zebra mussel filtration rate, its selective efficiency on various taxa, and phytoplankton growth rates interact to produce changes in the phytoplankton composition.
Changes in the cycling of phosphorus in the Upper Kis‐Balaton Reservoir following external load reduction
Authors: Istvànovics Vera , Somlyçdy Làszlç
Published: March 2003

1. The Upper Kis‐Balaton Reservoir was opened in 1985 to control eutrophication in Lake Balaton. High external TP load of the reservoir halved in 1991 due to improved sewage treatment. Annual TP retention efficiency of the reservoir dropped immediately from 50 to 20%, while the hypertrophic algal biomass started to decrease after 1995.

2. We examine mass balances of the main forms of P in three consecutive areas (1, 2 and 3) of the reservoir. Biologically available P (BAP) includes soluble reactive P and algal P, whereas the rest of TP is considered to be non‐available (nBAP).

3. Important pathways for P cycling varied in both space and time. A reduction in external load induced an enhanced net internal P load in the reservoir, and a variety of changes in the three areas. Thus, in the smallest Area 1, BAP retention dropped almost to zero, and the net sedimentation of nBAP decreased significantly. In Area 2 net BAP retention was characteristic before management, and net internal P load was observed afterwards. In Area 3 net internal P load has been substantial in some years ever since the reservoir was filled. The OECD model, corrected for internal P load, reasonably predicted the measured TP concentration in each area both before and after reduction in load.

4. The external load‐algal biomass curve differed in the three areas. An immediate response occurred in the upstream Areas 1 and 2, while a delay of 4–5 years was observed in Area 3. Different long‐term sediment dynamics may underlay these load‐response relations in the three areas. Intense sediment redistribution, however, sets a serious constraint to the simple calculation of a mass balance. An evaluation of the long‐term sediment dynamics is needed in order to predict future performance of the reservoir.
Dry mass–length relationships for benthic insects: a review with new data from Catamaran Brook, New Brunswick, Canada
Authors: Johnston Thomas A. , Cunjak Richard A.
Published: March 2003

1.___We summarized previously published mass–length relationships for aquatic insects, and determined the relationship between dry body mass and body length for eight genera and seven families of benthic insects from Catamaran Brook, New Brunswick, Canada.

2.___A power function was the most commonly used model in the earlier studies and best described the observed mass–length relationship for taxa from Catamaran Brook.

3.___Predicted mass at length was highly variable (coefficient of variation_≥ 25%) among models developed in different studies for the same family group. This variability presumably resulted from both variation in the methods used to construct the models, and in the natural spatio‐temporal and taxonomic variation in mass at length, although the relative contributions of these two sources cannot be determined from existing data.

4.___Several recommendations are made for the development and application of mass–length equations in future studies.
Zooplankton abundance in the River Danube, Austria: the significance of inshore retention
Authors: Reckendorfer W. , Keckeis H. , Winkler G. , Schiemer F.
Published: March 2003

1. Zooplankton density was examined in a free‐flowing section of the River Danube, Austria. Spatial and temporal patterns were used to identify mechanisms regulating zooplankton dynamics.

2. Zooplankton abundance differed significantly between sampling sites. These differences were the result of variations in water residence time within the habitats. Inshore habitats with low flushing rates act as storage zones for zooplankton.

3. The hydrological conditions govern the number and size of storage zones. In the study area, the highest availability of storage zones was observed at medium water level.

4. Total zooplankton numbers, total rotifer numbers and the densities of the most abundant rotifer taxa were significantly positively related to the availability of adjacent storage zones.

5. The present authors propose that inshore retention capacity, and the hydrological processes and mechanisms controlling the extent of inshore retention may be of major significance in regulated rivers with high water velocities, preventing substantial zooplankton growth in the main channel.
Temporal variability of stream bioassessments using benthic macroinvertebrates
Authors: Linke Simon , Bailey Robert C. , Schwindt John
Published: March 2003

1. When using benthic macroinvertebrate communities for bioassessment, temporal variation may influence judgement as to whether or not a site is degraded.

2. In a survey of sixteen reference and sixteen test sites in the upper Thames River catchment area (UTRCA) in south‐western Ontario, Canada, consistent differences between summer and winter samples were found for taxon richness (increase; P = 0.06) and the Family Biotic Index (decrease; P = 0.11). A bioassessment based on these results would indicate better water quality in the same streams in winter relative to summer. No consistent pattern of seasonal difference was detected for Simpson’s Diversity and Equitability, or percentage Dominant Taxon.

3. The Reference Condition Approach to bioassessment uses predictive modelling to explain variation in reference communities with the environmental conditions at these sites as predictors. The community at a test site is compared with that predicted by the model. Several predictive models were constructed using simple geographic and habitat characteristics (i.e. catchment area, distance to source, stream width, substrate and habitat diversity) as predictors. By including season of sampling in the models, we increased their predictive power and the ability of the bioassessment to detect degradation. The best results were achieved when separate predictive models were built for each sampling season.
Light absorption and photosynthesis of a benthic moss community: importance of spectral quality of light and implications of changing light attenuation in the water column
Authors: Schwarz Anne ‐Maree , Markager Stiig
Published: March 2003

1. The underwater light climate and benthic moss communities of Grane Langsø were investigated in May 1997 to determine the potential effects on benthic production of changing water column attenuation and spectral quality of light.

2. A reduction in water clarity in the lake since the 1960s was manifested as a marked increase in the attenuation of blue light, relative to red light, which can be attributed to increased dissolved organic carbon.

3. The biomass of the benthic moss community ranged from a maximum of 195 gDW m−2 at a depth of 4 m to 39 g DW m−2 at a depth of 10 m and Drepanocladus exannulatus contributed 70% of the biomass at all depths.

4. Absorption of PAR by D. exannulatus was maximal in the highly pigmented youngest parts of the plant and these correspondingly showed the highest rates of net photosynthesis. The absolute amount of chlorophyll‐a per g dry weight was greater at 10 m than 2 m, but the ratio of accessory pigments to chlorophyll‐a did not change. Deep growing plants did not show adaptation to changed light quality.

5. Increased attenuation of blue light and the resultant overall decrease in water clarity is likely to impact negatively on net annual production of benthic macrophytes of Grane Langsø. Any further increase in dissolved organic carbon concentration has the potential to markedly decrease the depth to which mosses grow by reducing the length of time in a year during which net photosynthesis occurs.
Fouling mussels (Dreissena spp.) colonize soft sediments in Lake Erie and facilitate benthic invertebrates
Authors: Bially Andrew , Macisaac Hugh J.
Published: March 2003

We conducted survey and transplant studies to determine whether colonization and residency on soft sediments by introduced, fouling mussels (Dreissena polymorpha and D. bugensis) were affected by physical disturbance, and whether Dreissena presence in turn influenced the diversity and population densities of other benthic invertebrates. Surveys revealed that colony density was typically higher at moderate depths than at shallower and greater ones. However, the largest, midsummer colonies and greatest coverage of sediments by mussels occurred at deeper sites.

Disturbance of transplanted colonies varied by depth and colony size, with deeper and larger colonies experiencing the lowest destruction rates. Colony destruction rate was positively correlated with current velocity adjacent to the lakebed.

Absence of mussel colonies at shallow sites was not determined by recruitment or substrate limitation, as recruit density was higher and sediment characteristics more suitable for postveliger settlement at shallow than at deeper sites. Rather, seasonal storms have much stronger effects in shallow than in deep water.

Mussel residency on soft sediment has profound effects on invertebrate biodiversity. Invertebrate species (taxon) richness and total abundance were positively correlated with mussel colony area. Mussel‐sediment habitat supported between 462 and 703% more taxa, and between 202 and 335% more individuals (exclusive of Dreissena) than adjacent soft‐sediment lacking mussels.

Results from this study illustrate that physical disturbance directly limits the distribution of mussels on soft sediments, and the diversity and abundance of other benthic invertebrates in consequence.
A geomorphic template for the analysis of lake districts applied to the Northern Highland Lake District, Wisconsin, U.S.A.
Authors: Riera JoaN. L. , Magnuson JohN. J. , Kratz TiM. K. , Webster Katherine E.
Published: March 2003

1. We tested the degree to which a lake's landscape position constrains the expression of limnological features and imposes a characteristic spatial pattern in a glacial lake district, the Northern Highland Lake District in north‐central Wisconsin.

2. We defined lake order as a metric to analyze the effect of landscape position on limnological features. Lake order, analogous to stream order, is based solely on geographical information and is simple to measure.

3. We examined the strength of the relationship between lake order and a set of 25 variables, which included measures of lake morphometry, water optical properties, major ions, nutrients, biology, and human settlement patterns.

4. Lake order explained a significant fraction of the variance of 21 of the 25 variables tested with ANOVA. The fraction of variance explained varied from 12% (maximum depth) to 56% (calcium concentration). The variables most strongly related to lake order were: measures of lake size and shape, concentrations of major ions (except sulfate) and silica, biological variables (chlorophyll concentration, crayfish abundance, and fish species richness), and human‐use variables (density of cottages and resorts). Lake depth, water optical properties, and nutrient concentrations (other than silica) were poorly associated with lake order.

5. Potential explanations for a relationship with lake order differed among variables. In some cases, we could hypothesize a direct link. For example, major ion concentration is a function of groundwater input, which is directly related to lake order. We see these as a direct influence of the geomorphic template left by the retreat of the glacier that led to the formation of this lake district.

6. In other cases, a set of indirect links was hypothesized. For example, the effect of lake order on lake size, water chemistry, and lake connectivity may ultimately explain the relation between lake order and fish species richness. We interpret these relationships as the result of constraints imposed by the geomorphic template on lake development over the last 12 000 years.

7. By identifying relationships between lake characteristics and a measure of landscape position, and by identifying geomorphologic constraints on lake features and lake evolution, our analysis explains an important aspect of the spatial organization of a lake district.
The seasonal dynamics and persistence of stream macroinvertebrates in Nepal: do monsoon floods represent disturbance?
Authors: Brewin P.A. , Buckton S.T. , Ormerod S.J.
Published: March 2003

1. The monsoon causes major flood events in some Himalayan streams, but their seasonal predictability might reduce the resulting disturbance. We assessed seasonal change in the benthos of 16 streams in central Nepal over a gradient of declining rainfall and increasing altitude from 600 to 3800 m. All sites were surveyed on four occasions, two in winter (November) and two pre‐monsoon (June), with additional sampling during the monsoon (August) at four low altitude sites. Invertebrate abundance, taxon richness and persistence were assessed at all sites, and density and meso‐habitat distribution at the four low altitude sites only.  2. Strong seasonal variation among invertebrates was confined primarily to streams at low altitude (600–800 m) where monsoon rainfall was greatest and catchments were dominated by terraced agriculture. At these sites, a significant reduction in benthic density (on average by 77%) and taxon richness (by 20%) occurred between the winter and pre‐monsoon periods, so that invertebrate numbers were already low before the monsoon. A further significant decline occurred in all meso‐habitats during the monsoon, but the change in density was small in absolute terms.  3. Persistence in rank abundance was equally low at all sites, but turnover in composition was significantly lower at sites in semi‐natural forest than in catchments managed for terracing or alpine pasture.  4. These data provide no evidence that monsoonal floods represent major disturbance, instead supporting the view that the ecological response might reflect an adjustment to predictable flow pattern. However, catchment land use in the Himalaya appears to be a significant source of ecosystem instability, and confounds the simple interpretation of monsoon effects.
Responses of a lake outlet community to light and nutrient manipulation: effects on periphyton and invertebrate biomass and composition
Authors: Bourassa Nathalie , Cattaneo Antonella
Published: March 2003

1. Experimental channels were installed at the outlet of a small Canadian Shield lake to study the role of light and nutrients on the regulation of periphyton and invertebrate biomass and taxonomic composition. Light (93% reduction) and nutrients (four‐fold increase of ambient total phosphorus (TP) concentration) were manipulated in a factorial design experiment.  2. Periphyton chlorophyll a (Chl a), measured four times during the 12‐week experiment, increased due to higher irradiance but was unaffected by phosphorus enrichment. Over the experiment, periphyton biomass was, on average, three times higher in open than in shaded channels.  3. Algal taxonomic and growth form composition were affected by light and phosphorus enrichment. The proportion of cyanophytes was significantly higher in unenriched, shaded channels (45%) compared to the three other treatments. Single cells and colonial forms were dominant in shaded channels, whereas filamentous and chain‐forming algae were prevalent in open channels.  4. Total invertebrate biomass remained unchanged over time and among treatments. At the beginning of the experiment, all the channels were dominated by Chironomidae and filter feeders (Simuliidae and Hydropsychidae). After 55 days, filter feeders became clearly dominant (60%) in all treatments except in the enriched, open channels where there was a significant shift in the functional group composition toward grazers (snails and oligochaetes). Among filter feeders, Simuliidae increased in shaded channels.
Mixotrophy in planktonic protists: an overview
Authors: Jones Roger I.
Published: March 2003

1. An overview is provided of the role of mixotrophic protists in plankton communities. Consideration of the importance of phagotrophy in the evolution of photosynthetic eucaryotes suggests that mixotrophy as a nutritional strategy can arise rather readily.  2. Mixotrophic protists actually present a spectrum of nutritional strategies. However, recognition of distinct groups of mixotrophs based on nutritional behaviour facilitates consideration of their functional role and of competitive interactions with other types of planktonic protists.  3. Consideration of the costs and benefits of mixotrophy as a nutritional strategy allows the development of several empirical predictions about the probable outcome of resource competition between mixotrophs and obligate phototrophs or phagotrophs. Existing results from laboratory and field experiments allow some of these predictions to be evaluated.  4. These results indicate that, under specified conditions, mixotrophs should represent an important link in the flux of materials through planktonic food webs. However, quantifying these fluxes remains a challenge for the future.
Benthic–pelagic coupling: a comparison of the community structure of benthic and planktonic heterotrophic protists in shallow inlets of the southern Baltic
Authors: Garstecki Tobias , Verhoeven Richard , Wickham Stephen A. , Arndt Hartmut
Published: March 2003

1. The taxonomic composition, abundance and biomass of heterotrophic protists (ciliates, heterotrophic flagellates (HF), rhizopods and actinopods) in the sediment and water column of shallow inlets of the Southern Baltic was studied under a variety of environmental conditions during 1996–1997. A shallow, highly eutrophic station and a deeper, less eutrophic station were compared.  2. Community biomass ranged from 0.12 to 0.34 μg C cm−3 in the water column and from 1.5 to 105 μg C cm−3 in the sediment. Heterotrophic protists dominated zooplankton biomass at both stations (73% and 84% mean contribution), while they were of minor importance within the zoobenthos. Expressed per unit area, benthic biomass contributed a significant part (44% and 49%) to the total heterotrophic protistan community at both stations.  3. Although the methodology for counting ciliates and HF was focussed on a high taxonomic resolution, the results reveal some general trends in the distribution of heterotrophic protists: protozooplankton biomass was dominated by flagellates (80% mean biomass contribution) at the shallow station and by ciliates (73% mean biomass contribution) at the deep station. In the benthos at both stations, ciliates were the dominant protozoans, followed by the hitherto little‐studied rhizopods (25% and 35% mean biomass contribution) and flagellates.  4. The degree of benthic–pelagic coupling differed between taxonomic groups. Benthic and pelagic communities of ciliates showed little taxonomic overlap. In contrast, many heterotrophic flagellate species were found both in the benthos and in the pelagic. These benthic–pelagic species contributed significantly to the biomass of HF in the water column. The planktonic rhizopod community consisted of a subset of those species found in the benthos.  5. The abundance of benthic and pelagic protists was positively correlated at the shallow station, but taxonomic data indicate that the direct exchange between benthic and pelagic communities was only partly responsible.
Impact of Bythotrephes longimanus on zooplankton assemblages of Harp Lake, Canada: an assessment based on predator consumption and prey production
Authors: Dumitru Cristina , Sprules W. Gary , Yan Norman D.
Published: March 2003

1. Following the 1993 invasion of Harp Lake by Bythotrephes longimanus a number of small‐bodied zooplankton declined dramatically compared to pre‐invasion densities, and some larger species increased.  2. To test whether these changes were caused by Bythotrephes, we measured its consumption of zooplankton in 1995 and compared this to production by the prey species with which it overlapped spatially and temporally.  3. On a seasonal basis Bythotrephes consumption was 199 mg dry mass (DM) m−2, or 25% of zooplankton production which was 783 mg DW m−2. However, for some species, such as the small copepod Tropocyclops extensus, consumption greatly exceeded production—this was one of the species noted to decline in Harp Lake following the appearance of Bythotrephes. By contrast, for the larger cladocerans Daphnia galeata mendotae and Holopedium gibberum, consumption was much less than production—these species were observed to increase following the invasion.  4. Our results thus support the hypothesis that zooplankton changes in Harp Lake were caused by Bythotrephes. We speculate that lower quality prey remaining in Harp Lake may lead to reduced densities of Bythotrephes, or impose strong selective forces that lead to new adaptations by this predator.
Colonization and production of macroinvertebrates on artificial substrata: upstream–downstream responses to a leaf litter exclusion manipulation
Authors: Baer Sara G. , Siler Edward R. , Eggert Susan L. , Wallace J. Bruce
Published: March 2003

1. Macroinvertebrate colonization dynamics were examined on artificial substrata in a stream with terrestrial litter inputs excluded, downstream of the litter‐exclusion treatment, and in a reference stream.  2. Short‐term examination of the rates of organic matter accrual and invertebrate colonization demonstrated significantly lower accumulation of leaf detritus and invertebrates in the litter‐excluded reach and a short distance downstream of that reach.  3. All major fractions of organic matter and invertebrates declined on artificial substrata during the 3‐year litter exclusion. Further, secondary production on artificial substrata in the litter‐excluded reach decreased from 6.2 to 1.5 g AFDM m−2 year−1 from pretreatment to the third year of litter exclusion, respectively.  4. Downstream, fine particulate organic matter on artificial substrata decreased during litter exclusion, and there was a significant reduction in colonization of collector‐filterers. Total secondary production downstream of the litter exclusion declined >70%, demonstrating that downstream colonization dynamics are linked to upstream detritus inputs and processing by stream invertebrates.
Assessing introgression of foreign strains in wild Atlantic salmon populations: variation in microsatellites assessed in historic scale collections
Authors: Martinez  , Dumas  , Beall  , Garcia‐Vazquez
Published: March 2003

1. Genetic variation at five microsatellite loci was analysed in a collection of scales (1970–97) sampled from Atlantic salmon adults returning to the Nivelle River (South France).

2. Native and foreign fish (from an allochthonous Scottish stock introduced into the river to increase population size) were clearly identified as all foreign individuals released in the river were physically marked.

3. Introgression of foreign genes into the native gene pool has occurred, although the reproductive success of foreign Atlantic salmon in the wild was lower than that of native individuals.

4. The utility of old scale samples for investigating the impact of foreign stocking on wild fish populations is demonstrated in this work.