Nutrient reduction and climate change cause a potential shift from pelagic to benthic pathways in a eutrophic marine ecosystem

The degree to which marine ecosystems may support the pelagic or benthic food chain has been shown to vary across natural and anthropogenic gradients for e.g., in temperature and nutrient availability. Moreover, such external forcing may not only affect the flux of organic matter but could trigger large and abrupt changes, i.e., trophic cascades and ecological regime shifts, which once having occurred may prove potentially irreversible. In this study, we investigate the state and regulatory pathways of the Kattegat; a eutrophied and heavily exploited marine ecosystem, specifically testing for the occurrence of regime shifts and the relative importance of multiple drivers, e.g., climate change, eutrophication and commercial fishing on ecosystem dynamics and trophic pathways. Using multivariate statistics and nonlinear regression on a comprehensive data set, covering abiotic factors and biotic variables across all trophic levels, we here propose a potential regime shift from pelagic to benthic regulatory pathways; a possible first sign of recovery from eutrophication likely triggered by drastic nutrient reductions (involving both nitrogen and phosphorus), in combination with climate‐driven changes in local environmental conditions (e.g., temperature and oxygen concentrations).     

Responses of Fish to Experimental Introduction of Structural Woody Habitat in Riffles and Pools

The declining condition of river systems associated with rapid development of human societies has led to substantial declines in fish diversity. One cause of decline is the loss of in‐stream Structural Woody Habitat (SWH), an important component of stream ecosystems, particularly as fish habitat. As a result there has been an increase in the number of rehabilitation programs that introduce SWH into rivers. This paper assesses fish responses to SWH introduction in riffles and pools in the Hunter River, eastern Australia, using a Multiple‐Before‐After‐Control‐Impact (MBACI) experimental and analytical design. In the riffle experiment, species richness was comparable among all control and treatment riffles across the entire study period. However, there were significant differences in assemblage structure, fish abundance, and biomass between control and treated riffles. The introduction of SWH (bank embedded deflector jams) appeared to create additional habitat which was utilized by one native fish species (Retropinna semoni—Australian smelt) and one alien species (Gambusia holbrooki—mosquito fish). In pools there were no significant changes in fish species richness, abundance, or biomass following introduction of SWH (pool jams). These findings have important practical and cost implications in terms of the design and implementation of rehabilitation strategies using SWH to restore fish assemblages in degraded streams.     

The chemistry of aquatic phosphate: inorganic processes in rivers

Phosphate levels in turbid rivers with low calcium concentrations are controlled by a solid ferric hydroxide-phosphate solution present in colloidal suspensions or suspended particulates. A chemical model, based on this behavior, is consistent with data from dialyzed suspensions of iron and phosphorus prepared in the laboratory as well as from the Amazon, Zaire, Orinoco, Sepik, Delaware, Hudson, Negro, and Mullica rivers. Data indicate that solid Fe/P ratios are related to solid activity coefficients by an exponential parameter, y, which represents the deviation of solid-solution from ideality. The model is mathematically consistent with Langmuir and Freundlich sorption isotherms under equilibrium conditions, and demonstrates that the isotherm parameters consist of a combination of selected constants and variables defined by solution theory. The reciprocal of the model parameter-y is shown to be equivalent to the exponential parameter in a Freundlich isotherm. The Langmuir parameter and Freundlich exponential parameter are related through the model parameter-y in systems at constant pH and ionic strength.From a presentation given at the Third International Workshop on Phosphorus in Sediments, Woudscholten/Utrecht, The Netherlands, September 30, 1991, under the auspices of: International Association of Theoretical and Applied Limnology, Limnological Institute (Royal Netherlands Academy of Arts and Sciences), Institute for Inland Water Management and Waste Water Treatment, and the Netherlands Institute for Sea Research.     

Use ofCladophora glomerata to monitor heavy metals in rivers

Methods were developed for the use ofCladophora glomerata to monitor heavy metal concentrations in flowing waters. At least under conditions without marked fluctuations in ambient metal concentration, there was no detectable difference in the metal concentrations of young plants between terminal 2-cm lengths of filament and whole plants. In order to establish the relationship between metal concentration in plant and that in water, 60 algal and water samples were analyzed from sites in northern England for Fe, Cu, Zn, Cd and Pb. Other environmental variables were measured at the time in order to assess their influence on metal accumulation. There were highly significant correlations for each of the five metals between concentrations in alga and water. The regression equations relating metal in alga to metal in water permit an unknown environmental metal concentration to be estimated from the algal concentration. Multiple stepwise regression analyses were used to indicate environmental factors which may influence metal accumulation; for instance, Fe appears to have a positive influence on Cu accumulation. In generalCladophora accumulates much less metal than bryophytes, but the slope relating metal in alga to metal in water is steeper, particularly for Pb. This means thatCladophora is especially useful where there is a need for a sensitive indicator of differences between sites or sampling occasions.     

Use of telemetry in the development and application of biological criteria for habitat hydraulic modeling

Neodiaptomus songkhramensis n. sp. from 86 temporary waters in the vicinity of Song Khram River in northeast Thailand is described and figured. It was found during May and June (rainy season) in Sakon Nakhon, Nakhon Pranom and Udon Thani Provinces. The new species usually co-occurs with 1–5 other diaptomids; the most frequently co-occurring species are Neodiaptomus blachei, Tropodiaptomus oryzanus, Neodiaptomus yangtsekiangensis, Dentodiaptomus javanus and Eodiaptomus phuphanensis.     

Are irrigation and grazing effects transferred,accumulated, or counteracted during plant recruitment?

How do effects from perturbations such as irrigation and grazing that have an impact at one stage of the recruitment process (e.g., seedling) affect performance at later stages (e.g., adult)? Such effects may be transferred to later stages without any further change (transferred effects), reinforced by a similar effect so that their importance increases (accumulative effect), or counteracted at later stages by an opposite effect (counteractive effect). We analysed the predominance of transferred, accumulative, and counteractive effects depending on (1) organization level (community, functional group, and species levels), (2) life cycle stage (seeds, seedlings, and adults), (3) grass/forb functional group (forbs versus grasses), and (4) seed mass (small-seeded species, medium-seeded species, and large-seeded species). The study was conducted in Alcalá de Henares, central Spain. During one annual cycle, we counted the number of readily germinable seeds (3,909 seeds: 2,156 forbs and 1,753 grasses), emerged seedlings (2,126 seedlings: 1,238 forbs and 888 grasses), and surviving seedlings up to reproductive adult status (917 adults: 217 forbs and 700 grasses) of all species (74 species) in a factorial field experiment under three different simulated rainfall and three seasonal sheep grazing regimes in a natural Mediterranean annual plant community composed of forbs (dicotyledonous herbs) and grasses (monocotyledonous herbs). Our main results were: (1) the proportion of transferred and counteractive effects was similar between the three studied organization levels (community, functional group, and species levels); (2) we detected many more counteractive effects in the seedling stage than in the adult stage; (3) we did not find more counteractive effects in forbs versus grasses, but transferred effects were more frequent in grasses; and (4) we found more counteractive effects with decreasing seed mass and more transferred effects with increasing seed mass. Our main conclusions were that the prevalence of counteractive effects reduces the sensitivity of the whole recruitment process and enhances the relative importance of the final stage (adults). The prevalence of transferred effects makes the recruitment process more predictable and more dependent on the early stages (seeds and seedlings).     

Recent sedimentological conditions in the delta of the river Kyrönjoki,Western Finland

The estuary of the River Kyrönjoki was echo sounded and, on the basis of these results the delta was delimited and areas of erosion and accumulation were separated. Samples of the sediment surface (0–2 cm) from 65 sites in the estuary were analysed for organic content, C, N, P, Ca, Fe, Mn, Pb, Cu, Zn, Cd and Hg. On the basis of the chemical analyses the delta was divided into two parts according to the sedimentary conditions: (I) In the inner part, where acid fresh water of c. pH 5.5 from the river is predominant, the conditions were reducing. The Fe/Mn ratio of the sediment was high. The heavy metal content was usually low. The Hg content in the organic compounds of the sediment was, due to the acidity, higher than in the outer part. (II) In the outer part of the delta, where neutral brackish water of pH c. 7.0 dominates, the sedimentary conditions were mostly oxidizing. The Fe/Mn ratio was very low. The heavy metal and organic content was mostly high in the accumulation areas. This is because the fine-grained allochthonous material, which has a high heavy metal and organic content, flocculates and precipitates when it reaches the brackish water of the Gulf of Bothnia, and the salinity and the pH are higher.