Seasonal changes in chemistry,algal populations,chlorophyll a and photosynthetic activity in the R. Delimi,Jos Plateau,Nigeria

Seasonal studies were undertaken (August 1983–July 1984) toevaluate the algal distribution and ecology of the RiverDelimi, Jos Plateau, Nigeria at three stations: upstream, withlittle allochthonous input; midstream, receiving heavy domesticaffluents; downstream. A wide amplitude of variability wasdiscernible in physicochemical factors, and photosyntheticactivity and abundance of suspended algae in relation to thedegree of pollution along the first 7-km stretch of the river.Conductivity (µS cm^-1) showed a 9-fold increase fromupstream (x = 35) to downstream. The standing crop(Chl a) fluctuated markedly, with 6-times higher valuesfor a polluted midstream site (x = 18.2 mg m^-3). Production rate measurements (mg C m^-3 d^-1) showed significant variability with relatively lowvalues (x = 610) recorded for the upstream andhighest (x = 1050) at the midstream site. Apositive correlation was obtained between production rates andChl a for the polluted midstream zone.     

The impact of the Scheldt input on the trace metal distribution in the Belgian coastal area (results of 1981–1983 and 1995–1996)

Offshore fluxes of Cu, Zn, Cd, Pb and Hg were calculated based onresidual flow patterns and salinity gradients along the Belgian coast. Theresidual flow lines along the Belgian coast are more or less parallel to thecoast except in the area where the north-easterly flowing watermass comingfrom the Channel encounters the south-westerly-oriented Scheldt outflow,forming a residual hydrodynamical front. From the steady-state salinitypattern, diffusion coefficients perpendicular to the residual flow werededuced; they ranged from 21 to 108 m² s^-1. Offshore fluxes of dissolved and particulate trace metals based on diffusiveand mixing processes are calculated. The steady state profiles of dissolvedmetals show a dilution effect in the coastal waters, reaching an almostconstant concentration in the marine watermass in the 1981–1983dataset. The ratios of the Scheldt input of trace metals to the totaldissolved offshore flux vary from 38 to 55% (1981–1983),depending on the kind of metal, and from 55 to 91% (1995–1996).The ratio of the Scheldt input to the dissolved metal flow parallel to thecoast, is in both periods (1981–1983 and 1995–1996), smallerthan 1%. The steady-state concentration profiles of particular metalsversus salinity are fairly constant in the coastal-estuarine and marinewatermasses, but decrease very abruptly from the first to the secondwatermass. Assuming a conservative behaviour of the particular metals,offshore fluxes and the resulting concentration increases agree fairly wellwith the observed values. The ratios of the Scheldt input to the particulatetrace metal offshore flux vary between 30 to 46% (1981–1983)and 13 to 37% (1995–1996). The contribution of the Scheldtestuary to the flows parallel to the coast ranges from 1.6 to 2.9%(1981–1983) and from 0.6 to 1.6% (1995). This revised version was published online in July 2006 with corrections to the Cover Date.     

Life history and species composition of the damselfly assemblage along the urban tract of a river in central Italy

The species composition of the damselfly assemblage and the life history patterns of two Coenagrionidae (Ischnura elegans and Cercion lindeni) were investigated along the urban tract of a river characterized by increasing organic pollution. The assemblage was dominated by generalist species, usually recorded in lentic habitats, rather than by typical riverine species and the proportion of the latter decrease at the most polluted sites. At the end of Winter, the mean size and in star distribution were different between the sampling sites showing that the life history of both species examined were influenced by a degradation of the environmental quality. A longer reproductive period, absence of diapause, and tolerance of low oxygen concentration appear to be key factors that allow generalist species I. elegans and C. lindeni to predominate at the polluted sites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Progressive changes in the structure and dynamics of the phytoplankton community along a pollution gradient in a lowland river — a multivariate approach

The phytoplankton of the River Lujan (Buenos Aires, Argentina) was studied for a period of 18 months, together with physical and chemical variables, in relation to a pollution gradient. 167 taxa were recorded within a seasonal succession characterized by dominance of diatoms with a brief summer green algae facies. A combination of several biotic indices and multivariate analysis was employed to assess the impact of pollution on the phytoplankton community. The biotic indices used were species diversity and richness, algal quotients (green algae/diatom ratio, Centrales/Pennales ratio) and the SD succession rate index. Multivariate procedures included cluster analysis and ordination by PCA of both species and samples, stepwise discriminant analysis and multiple discriminant analysis of variance (MANOVA). Results indicate that community dynamism is attenuated at the more polluted sites, concomitant with an increased predominance of a broad-tolerance algal assemblage, co-dominated by Cyclotella meneghiniana and Nitzschia stagnorum. The changes in the community structure and dynamics described herein involved alterations in the distribution and relative proportions of the algae, rather than modifications in the basic species composition. These changes may not be readily detectable by methods which over-simplify the ecological information, such as systems of indicator species and biotic indices, designed to assess the degree of pollution. The suitability of multivariate analysis and biotic indices in river phytoplankton studies is further discussed.     

Substrate selection and seasonal variation in densities of invertebrates in stream pools of a tropical river

Spatial distribution and seasonal variation in densities of the invertebrates were investigated for a year in three stream pools of a South Indian river. The effects of season, substrate type and water depth on the distribution were analyzed. Substrate type and season influenced the invertebrate distribution the most. Leaf packs harboured most of the organisms followed by macro-algal substrate and sand. The lowest densities were observed on rocky substrates and in the water column. Rocky substrates in shallow water supported higher densities of total invertebrates than deeper areas. Chironomid larvae dominated all benthic substrates throughout the year. Of the 19 invertebrate taxa studied, 6 showed no seasonality in densities, and most of the rest showed their highest densities in the pre-monsoon period and lowest in the SW monsoon or post-monsoon periods. However, in two of the three pools, the densities of total invertebrates were highest during the post-monsoon period with secondary peaks in the pre-monsoon period.     

Transport of coarse particulate organic matter in an Idaho river,USA

I investigated organic matter transport in the Henry's Fork of the Snake River, Idaho, USA, from August 1987 to November 1988. Mean discharge during the study was 15 m³ s^–1. Screens were used to sample very coarse (> 6 mm) transported aquatic macrophyte material (VCTMM). Drift nets were used to sample coarse (1–6 mm) and fine (0.25–1 mm) transported particulate organic matter (CTOM and FTOM). Mean monthly concentration of VCTMM was 0.064 mg AFDWl^–1 and was significantly higher than CTOM (0.024 mg AFDW l^–1) and FTOM (0.036 mg AFDW l^–1). VCTMM concentration was highest in December (0.163 mg AFDW l^–1) and lowest in May (0.018 mg AFDW l^–1). The sample position along a transect across the channel had a significant effect on the amount of transported organic matter collected in many months. The concentration of debris from individual species tracked the standing stock of that species during the growing season. In Fall, a dramatic increase in VCTMM corresponded to a decrease in macrophyte standing stock. FTOM and CTOM concentrations were highest in January (CTOM: 0.048; FTOM: 0.111 mg AFDW l^–1), lowest in November 1988 (<0.006 mg AFDW l^–1), were not correlated with discharge, and were inversely correlated with the standing stocks of macrophytes upstream, probably because macrophyte beds influenced the retentiveness of the channel. Standing stock of aquatic macrophytes was highest in September–October (5.2 kg wet weight m ^–2) and lowest in February (1.7 kg wet weight m^–2). Annual movement of particulate organic matter past the sampling point was about 45 000 kg AFDW, of which 21 000 kg was VCTMM, 8 000 kg was CPOM, and 16 000 kg was FPOM.     

Macrobenthic community structure before and after pollution abatement in the Neches River estuary (Texas)

Macrobenthos and physicochemical conditions in the lower 39 km of the Neches River estuary were studied from August, 1984 to May, 1985. The results were compared with data collected in 1971–1972. Between 1972 and 1984 the permitted BOD waste load in the tidal Neches River was reduced from 123 125 kg d to 8717 kg d. River discharge and dissolved oxygen concentrations were consistently higher and salinity was lower, during the same seasons, during the 1984–1985 study. A total of 50 taxa of macrobenthos were collected in 1971–1972 and 104 taxa were collected in 1984–1985. The numbers of taxa per collection at each station in 1984–1985 were at least twice those found in 1971–1972. Minimum densities in 1984–1985 were much higher than the maximum densities in 1971–1972 at all stations. Patterns of dominance, Sorenson's similarity index, and diversity ( ) values indicated improved water quality in 1984–1985. Statistical analysis of macrobenthic diversity indicated significant differences between upper estuary and lower estuary stations in 1971–1972. No significant differences were found in 1984–1985. Significant differences in numbers of taxa, macrobenthos densities, and values between the two studies were found. Reductions of waste loads, increased river discharge, and deepening of the navigation channel were among the factors that probably contributed most to the changes in community structure of the macrobenthos observed.     

Epilithic diatom community response to years of P04 fertilization: Kuparuk River,Alaska (68 N Lat.)

An arctic river was fertilized continuously through the ice-free season with phosphoric acid beginning in 1983. The epilithic diatom community increased in biomass in the first two years in response to the added limiting nutrient (Peterson et al., 1983). The diatom community switched from one dominated by Hannea arcus to one dominated by species of Achnanthes and Cymbella. The immediate responses to the P-addition were decreases in both the Shannon diversity and evenness indices. By the second year, the community diversity increased downriver reaching maximal species richness (110–127 spp). In 1985–1987, the epilithic algal biomass decreased an order of magnitude with both whole-river PO₄ (1985, 1987) and PO₄ + NH₄ addition (1986). In the 5th summer of fertilization, the reduction in biomass was clearly caused by a numerical increase of grazing, refugia-building chironomids (Orthocladiinae, primarily) (Gibeau, 1991; Gibeau, Miller, Hershey, in prep.). We assume the algal biomass reduction in the 3rd and 4th years was similarly caused by grazers with a two year time lag in the numerical response of these monovoltine species. The evenness of the community increased in 1986 as if it might have been grazed; however the number of immigrants was reduced. The community became dominated by Eunotia, Cymbella and Achnanthes, species either fast growing or more prostrate, as the erect species of Hannea Diatoma, and Fragillaria declined. A detrended correspondence analysis of the temporal and spatial diatom samples in species space (186 spp.) showed that the largest variation in the community was between years and less variation was associated with river fertilization. Samples from bioassay tubes run by Peterson et al. (1983) in the Kuparuk River showed P and N + P limitation as found in the river in 1983–84. Like the river samples, the largest change in the diatom community occurred between 15 and 25 day samples, more than that induced by fertilization. Diatoms sampled from all treatments taken at day 25 were more similar to one another than those sampled at day 15. Diatoms colonizing glass slides used in the bioassay tubes were dominated by Achnanthes linearis and Cymbella minuta. Of the 84 species found in bioassays, 26 species were present in all river samples for 4 years. Differences in the communities discriminated by multivariate methods were cause by changes in rare species and abundance patterns of common species.     

Correlations between chironomid pupal skin collections and habitats recorded from a chalk river

Chironomid pupal skins were collected during one year from three sites along a chalk stream in southern England. The sites had similar chemistry and discharge but differed in their current and temperature regimes. Substrate composition upstream of each collection point was surveyed during spring, summer and autumn. Proportions of macrophytes and sediments were compared with proportions of chironomid species and trophic groups. Seasonal changes in substrate and pupal skin collections were correlated by classification and direct ordination techniques. The distribution of chironomid species were indicated as being significantly related to the recorded substrate data.     

Water quality changes of the river Meuse assessed by chironomid pupal exuviae

The chironomid fauna of the river Meuse in Belgium and The Netherlands was studied in order to determine spatial water quality variations. Because the river Meuse serves as a source for drinking water production for about five million people in Belgium and The Netherlands, water quality is an essential factor to be studied. From 1984 till 1989 pupal exuviae were collected. Water quality was assessed by several indices and autecological information of the various taxa. With regard to pollution the general trend through the years was the same. The most upstream sampling site, in the Belgian Ardennes, was relatively unpolluted. As a result of untreated industrial and municipal wastewater discharges in the Liège region, water quality deteriorated dramatically at the Dutch border. At the most downstream sampling site, near the river mouth, a slight recovery was observed. Apart from organic and inorganic pollution other environmental factors such as flow velocity and the nature of the riverbed, also play a significant role in the occurrence of the organisms. It is concluded that protective measures need to be taken to restore the affected river ecosystem.The editors deeply regret to have to announce that Nicole Frantzen died in a train accident on 30 November 1992.