Biomanipulation in shallow lakes in The Netherlands: an evaluation of 18 case studies

Eighteen shallow lakes in The Netherlands were subjected to biomanipulation, i.e. drastic reduction of the fish stock, for the purpose of lake restoration. The morphology and the nutrient level of the lakes differed, as did the measures applied. In some lakes biomanipulation was accompanied by reduction of the phosphorus loading. In all but two lakes, the Secchi disk transparency increased after the fish removal. Eight lakes (no phosphorus loading reduction, except for one lake) showed a strong and quick response to the measures: the bottom of the lake became visible (`lake bottom view'') and there was a massive development of submerged macrophytes. In eight other lakes the water transparency increased, but lake bottom view was not obtained. In the biomanipulated lakes the decrease in total phosphorus and chlorophyll aand the increase in Secchi disk transparency were significantly stronger than the general trend occurring in Dutch lakes where no measures had been taken. The improvement in the Secchi depth and chlorophyll awas also stronger than in lakes where only the phosphorus load was reduced. The critical factor for obtaining clear water was the extent of the fish reduction in winter. Significant effects were observed only after >75% fish reduction. Success seems to require substantial fish manipulation. In such strongly biomanipulated lakes, wind resuspension of the sediment never prevented the water from becoming clear. No conclusion can be drawn with respect to the possible negative impact of cyanobacteria or Neomysison grazing by Daphniaand consequently on water clarity. In all lakes where there had been a high density of cyanobacteria or years with a high density of Neomysisother factors such as insufficient fishery may explain why lake bottom view was not obtained. In all lakes with additional phosphorus loading reduction the fish stock has been reduced less drastically (15–60%). In these lakes the effects on transparency were less pronounced than in the lakes with > 75% fish removal. Daphniagrazing seems responsible for spring clearing in all clear lakes but one. In three lakes the reduction of benthivorous fish also increased the transparency. The factors that determine water clarity in summer are less obvious. In most clear lakes a low algal biomass coincided with a macrophyte coverage of more than 25% of the lake surface area. However, it was not clear what mechanism caused the low algal biomass in summer, although inorganic nitrogen concentrations were regularly found to be very low. Daphniagrazing in open water seemed to be of little importance for suppressing the algal biomass in summer. Although in most lakes the total phosphorus concentration decreased after the biomanipulation, the dissolved phosphorus concentration remained too high to cause phosphorus limitation of the algal growth. In four out of six clear lakes for which there are long-term data the transparency decreased again after 4 years. In one lake with lower nutrient levels the Secchi disk transparency increased over the years. However, the number of lakes with low nutrient levels is too small for conclusions to be drawn regarding the impact of nutrient levels on the stability of the clear water state

WebGR – storing images of biological material and creating a framework to promote the implementation of sound statistical analysis in age calibration

The objective of the WebGR project is to develop a set of web services to support the organisation and data analysis of calibration workshops, both for age and maturity information, implemented in a coherent tool installable as a website. The website consists of a repository of images, a set of web forms to run a calibration exercise online, a reporting module with the most common statistical analysis and import/export modules to manage images and results. The software has a creative commons license (Open Source) to promote transparency, technology transfer and peer review; and will allow the scientific community to get involved in further developments, like linkage to statistical analysis engines, or any other specific features. The usage of WebGR to carry out calibration workshops will promote the application of sound statistical analysis to design the experiment and compute workshop results. The results are extracted in a standard format that can be easily sent to scientists doing assessments. Keywords: WebGR, calibration workshops, age, maturit

Study on stomach content of fish to support the assessment of good environmental status of marine food webs and the prediction of MSY after stock restoration

The current reform of the Common Fisheries Policy anticipates more extensive use of long-term management plans which are consistent with the ecosystem approach to fishery management. Both long term management plans and estimates of the fishing mortality providing MSY are particularly sensitive to changes in natural mortality, and a prerequisite for estimating natural mortality correctly is the accurate knowledge of species interactions for application in multispecies models. The use of historic data in the multispecies models has so far been limited by the need for data to represent a full spatial coverage. However, the recent model developments have made it possible to integrate regional samples of stomach content data into the multispecies and ecosystem models. It is hence no longer necessary to have complete spatial coverage in a given year, before new stomach data can be included into the models. This provided a unique opportunity to utilize the vast amount of historic data available at individual fisheries research institutes. DTU Aqua together with 8 partners from the Baltic and North Sea has conducted a stomach collection and analysis project in order to (i) include all appropriate historical stomach content information into the Baltic and North Sea stomach content databases, (ii) conduct stomach content analyses of new cod stomachs collected in the Baltic Sea, to support our knowledge of the spatial and temporal stability of cod preferences, and (iii) conduct stomach content analyses of whiting stomachs collected in the Baltic Sea and grey gurnard, mackerel and hake collected in the North Sea to support our knowledge of potentially important predators for which the diet is presently poorly known or is expected to have changed significantly since the last sampling efforts in this area. All existing and new samples and data are included in the final product of this tender, i.e. a common stomach database in ICES exchange format, which is going to be made available to the scientific community via ICES. This database can then be used to re-estimate multispecies reference points such as FMSY of the different fish species. Within the Baltic Sea, efforts were focused on stomach content analyses of Baltic cod as the most abundant piscivorous fish in this ecosystem. The latest data in the cod stomach content database are from 1993. Since then, the Baltic has changed markedly. Both abundance and spatial distribution of cod and its major fish prey, herring and sprat have changed. Furthermore, due to the occurrence of extended hypoxic areas on the sea-bed, the availability of benthic food may also have changed dramatically. In addition to cod, a limited number of whiting stomach samples from the westernmost areas of the Baltic were collected and analyzed, as this species is potentially another important piscivorous predator in these regions. Before the start of the project, stomach content data for 49476 Baltic cod from the period 1977-1993 were available. The initial estimate in the proposal for the present stomach tender was that this number could be increased by ca. 170% during the course of the project. However, considerably more data have been made available during the project, especially due to the initial underestimation of the available number of historic data. The new data that have been made available increased the number of data to 255% of the initially available data. Since the focus of this tender was on the Baltic Sea, only relatively limited resources were allocated for work in the North Sea. Thus, in the North Sea, a ‘most benefits for least cost’ strategy was employed. This involved (1) the collection and analysis of new stomachs from selected species which were judged to provide the most relevant new knowledge for multispecies modeling and (2) the transformation of existing historic data into ICES exchange format. Stomachs of 1907 grey gurnard, 517 hake and 702 mackerel have been analyzed, corresponding to 119, 65 and 44 % of target values, respectively. In contrast to the Baltic Sea part of the project, that already had a large number of samples available from the beginning, the North Sea partners had to wait with initiating the stomach analyses until sufficient numbers of samples had become available, and hence the number of analyzed stomachs was not as predictable as for the Baltic Sea. Furthermore, the conversion of historic data from the DAPSTOM database has provided a large number of additional data for these three target species as well as a multitude of other species (> 207.000 data records for 184 species). Both for the Baltic and the North Sea regions, the new data have already been used and generated great interest in the scientific community. In order to facilitate the application of these data, DTU Aqua will beyond this project make data extracts available on request and collaborate with ICES to make the database available on the ICES data portal as soon as possible

MYCORRHIZAE AND NUTRIENT CYCLING IN NATURAL FOREST ECOSYSTEMS

The widespread occurrence of mycorrhizae in nature and their importance in the mineral nutrition of almost all plants has been extensively documented but despite this mycorrhizae have not been included in nutrient cycling studies of forest ecosystems. This neglect may be due to a failure on the part of researchers to recognize the functional differences between mycorrhizae and roots and to the labour-intensive effort needed to study mycorrhizae. Biomass or surface area of mycorrhizae must be measured before information on ion absorption by mycorrhizae can be applied to forest ecosystems. A full assessment of the importance of mycorrhizae in nutrient cycling also requires data on mycorrhiza production, senescence, and decomposition. Few mycorrhiza studies have provided such data. Consequently, our information on nutrient cycling is derived from fine root (≥ 5 mm in diameter) data which may or may not include mycorrhizae. Recent studies have shown that most of the organic input to the decomposition process results from fine root production. Fine root mortality and decomposition is also more important than other mechanisms for returning nitrogen immobilized in vegetation to the soil in both deciduous and coniferous forests. A recently completed study indicates that mycorrhizae account for 50% of the annual throughput of biomass and for 43% of the nitrogen released annually in a Douglas fir ecosystem. These transfers are five times larger than the releases from litterfall or litter decomposition. Clearly, the study of mycorrhizal nutrient cycling is in an embryonic state and considerable additional research is needed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65533/1/j.1469-8137.1980.tb03189.x.pd

Epibenthic diversity in the North Sea

In 1999 the epibenthic fauna of the North Sea was investigated using the 3rd quarter 'International Bottom Trawl Survey' of five European countries. Altogether 241 stations were sampled covering 143 ICES rectangles. The objectives of the projects were (i) to analyse epibenthic diversity patterns in the North Sea, (ii) to identify the spatial distribution of faunal communities and (iii) to relate environmental factors as well as fishing effort to species diversity. Epibenthic fauna was clearly divided between the southern North Sea and the central-northern North Sea, roughly along the 50 m depth line. The separation was based on an overall higher number of species in the central and northern North Sea and a change in the species composition from north to south. Sessile fauna including erect, branching species like bryozoans and hydrozoans were particularly diverse along a corridor in the central-northern North Sea between 56° and 58°N, coinciding with the area between the 50 m and 100 m depth line. Cluster analysis, based on the structure of the community, confirmed the north-south gradient found for species diversity. Separation of clusters was driven to a great extent by species occurring predominantly or exclusively north of the 50 m contour line. Few species were exclusive to the south, but a number of scavenging species were found here more frequently and in higher numbers. Depth was positively correlated with the diversity of free-living fauna, whereas the type of sediment showed no significant relationship with variations in numbers of species. Beam-trawling effort was negatively correlated with the diversity of sessile fauna