A Study of Surface Water Quality in Macedonia,Greece: Speciation of Nitrogen and Phosphorus

A 2-yr (1997–1998) survey aiming at the establishment of national data bases concerning the quality of surface waters has been conducted in the major river systems of Macedonia, N. Greece. This paper presents the physicochemical parameters(pH, conductivity, total suspended solids, temperature and DO),the organic pollution parameters (BOD₅, COD) and the major N and P species (NO₃ ^-, NO₂ ^-,NH₄ ⁺,organic N, orthophosphates and total P) determined at 25 sampling sites located on main rivers, tributaries, streams andditches that drain the major rural, agricultural, urban and industrial areas of N. Greece. Use of multivariate statistics is also made to identify the principal factors which influencethe chemistry of the water in individual river systems.The eutrophication status of the examined systems was evaluatedby means of N/P ratios. Mean N/P ratios showed large variationsamong sampling sites ranging from potential N- to P-limitationconditions. N/P ratios at particular sampling sites showed also great temporal variability thus suggesting temporary states of N- or P- limitation. Most frequently, highest ratio values wereobserved during winter and early spring. Comparisons are made between data from this study region and literature on rivers elsewhere.     

Estimating uncertainty in fish stock assessment and forecasting

A variety of tools are available to quantify uncertainty in age‐structured fish stock assessments and in management forecasts. These tools are based on particular choices for the underlying population dynamics model, the aspects of the assessment considered uncertain, and the approach for assessing uncertainty (Bayes, frequentist or likelihood). The current state of the art is advancing rapidly as a consequence of the availability of increased computational power, but there remains little consistency in the choices made for assessments and forecasts. This can be explained by several factors including the specifics of the species under consideration, the purpose for which the analysis is conducted and the institutional framework within which the methods are developed and used, including the availability and customary usage of software tools. Little testing of either the methods or their assumptions has yet been done. Thus, it is not possible to argue either that the methods perform well or perform poorly or that any particular conditioning choices are more appropriate in general terms than others. Despite much recent progress, fisheries science has yet to identify a means for identifying appropriate conditioning choices such that the probability distributions which are calculated for management purposes do adequately represent the probabilities of eventual real outcomes. Therefore, we conclude that increased focus should be placed on testing and carefully examining the choices made when conducting these analyses, and that more attention must be given to examining the sensitivity to alternative assumptions and model structures. Provision of advice concerning uncertainty in stock assessments should include consideration of such sensitivities, and should use model‐averaging methods, decision tables or management procedure simulations in cases where advice is strongly sensitive to model assumptions.