No abstract

Features of the colonial diatom Didymosphenia are reviewed, especially D. geminata. Although there is a long record of its occurrence in north temperate regions, mass growths have been reported much more widely in recent years. Contrary to some statements in the literature, there are also reliable older records for the southern hemisphere, though the first report of mass growth was in New Zealand in 2004. The annual cycle of morphological changes in D. geminata in northern England, and probably elsewhere, includes a winter period when motile cells are attached to the substratum followed by spring when stalks start to develop. These raise cells into the water column and provide a site for phosphatase activity. Environmental factors associated with success include absence of extreme floods, high light, pH above neutral and nutrient chemistry. D. geminata often, but not always, occurs in waters where the N:P ratio is high for much of the year, but the key factor is the ratio of organic to inorganic phosphate. D. geminata thrives where organic P is predominant and the overall P concentration is low enough for organic P to be an important P source. It is unknown whether organic N can be used. Environmental changes increasing the relative importance of organic P are likely to favour D. geminata. Likely examples are increased N:P due to atmospheric N deposition and changes in form and seasonality of P release from organic-rich soils due to climatic warming. The nutrient chemistry of deep water released from dams to rivers also needs investigation. To what extent are genetic changes occurring in response to environmental changes and are new ecotypes spreading round the world? In spite of many adverse reports about D. geminata, such as detached mats blocking water pipes, there is still doubt about the extent to which it causes problems, particularly for fish. There have been few adverse effects on migratory salmonids in Europe and North America, but at least one report of harm to a brown trout population in USA. In New Zealand, it has caused serious problems for water sports, although it remains open to question how much adverse effect it has had on fish populations. If the presence of microcystins in or associated with D. geminata, as indicated recently for two populations, proves to be widespread and at sufficiently high concentration, their possible accumulation in fish requires study. Where control is required, this could Handling editor: Pierluigi Viaroli be achieved by enhancing the ratio of inorganic to organic phosphate in the water early in the growth season. Practical ways to achieve this are suggested.

Phosphatase activities and environmental features were characterized for 12 terrestrial and aquatic mosses in upland northern England, along with four species sampled from subarctic Sweden. Phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities of shoot tips were measured using para -nitrophenyl phosphate ( p -NPP) and bis-p NPP. All species showed PMEase activity, but not all showed PDEase activity. The mean pH optimum was 5·0 for PMEase and 5·7 for PDEase

A standard method is given for describing plant communities in fast-flowing streams and rivers. This involves the selection of a representative lO-m length of stream (termed a reach) which can be re-located exactly. Various estimates of the relative areas covered by different physiognomic forms and the relative proportions of species present are then combined to give a "semi-quantitative" account, which can be compared with data collected at other sites or times. The method has been applied to six sites on the River Tees system. A site below Cow Green reservoir with markedly regulated flows, differs from the others in various ways. Bryophytes show a greater cover here than elsewhere throughout the year and a conspicuous cover of Phormidium occurs in autumn. Filamentous green algae of many different species are abundant at this site in late summer and early autumn. At the most downstream site where such seasonal growths also occur, a single species, Cladophora glomerata, is dominant. Conspicuous spring diatom 'bursts' occurred at the two upstream sites and a month later at the two downstream sites.

This introduces a series of five papers describing physical, chemical and biological features of Bangladesh deepwater rice-fields. Flooding occurs typically from late June to early November. There were two distinct flood peaks (late July, mid-September) at the two main research sites during each of the years 1981-86, with maximum depths reaching about 1.5-2.0 m. Temperature showed a narrow range, with values for the upper part of the water column during July-October seldom outside the range 29-35 "C. Data for PAR (Photosynthetically Active Radiation) obtained with two types of sensor (4 n and cosine) are summarized, with examples of profiles and changes throughout the day. Water chemistries sampled from 13 different locations at various times during 1981-1983 are compared for: conductivity, pH, total alkalinity, O,, major metals (Na, K, Mg, Ca, Mn, Fe), minor metals (Co, Ni, Cu, Zn, Cd, Pb), NH4-N, NO2-N, NO3-N, filtrable reactive P, filtrable organic P, non-filtrable P, C1, SO4-S, reactive-Si.Overall the concentrations of aqueous N and P may be regarded as providing a mesotrophic environment for algae, but with relatively low N:P. Filtrable organic P was at least as important as filtrable reactive P, with mean values at the various locations from 0.020 to 0.036 mg 1-' P for the former and 0.005 to 0.038 mg 1-' for the latter. Measurements of N and P in the water column at any one time typically gave values (expressed per unit area) slightly lower than totals for these elements deposited in sediments throughout the whole season. At least until the end of September, the upper part of the water column was almost always oxic during the daytime and occasionally supersaturated with 0, in the afternoon. However DWR fields in south Bangladesh (Phaltita) visited on a sunny day in late September had waters which were almost entirely anoxic even in the afternoon: channels were lacking here and rotting rice stems from an earlier crop were widespread.