Using geophysical information to define benthic habitats in a large river

1. Most attempts to describe the distribution of benthic macroinvertebrates in large rivers have used local (grab‐scale) assessments of environmental conditions, and have had limited ability to account for spatial variation in macroinvertebrate populations. 2. We tested the ability of a habitat classification system based on multibeam bathymetry, side‐scan sonar, and chirp sub‐bottom seismics to identify large‐scale habitat units (‘facies’) and account for macroinvertebrate distribution in the Hudson River, a large tidal river in eastern New York. 3. Partial linear regression analysis showed that sediment facies were generally more effective than local or positional variables in explaining various aspects of the macroinvertebrate community (community structure, density of all invertebrates, density of fish forage, density of a pest species –Dreissena polymorpha). 4. Large‐scale habitats may be effective at explaining macroinvertebrate distributions in large rivers because they are integrative and describe habitat at the spatial scales of dominant controlling processes.     

Using a 1-D mixing model to assess the potential impact of year-to-year changes in weather on the habitat of vendace (Coregonus albula) in Bassenthwaite Lake, Cumbria

1. Bassenthwaite Lake in Cumbria is one of only two English lakes containing a population of vendace (Coregonus albula). The spatial distribution and survival of this fish is strongly influenced by the temperature and oxygen content of the water. In summer, this fish moves into deeper, colder water but avoids areas where the oxygen content is low. 2. In recent years, there has been a dramatic decline in the number of vendace found in the lake, a trend that may have been exacerbated by a succession of warm summers. Bassenthwaite only becomes stably stratified during calm, warm periods when a significant proportion of the deep water becomes anoxic. 3. Here, a one dimensional (1‐D) process‐based temperature‐oxygen model is used to simulate the year‐to‐year variations in the severity of these ‘extreme events’. The model is validated using field measurements acquired in the 1990s and used to predict the range of depths accessible to the vendace. 4. An empirical, weather‐driven model is then used to ‘hindcast’ the mixing characteristics of the lake in the 1980s and estimate the proportion of the habitat lost during warm, calm summers. These simulations show that periods of stable thermal stratification have become increasingly common in recent years. In the 1980s, only one ‘extreme event’ was identified but four such events were recorded in the 1990s. 5. The results are discussed in relation to the conservation status of the species and the potential effect of climate change on its survival in the English Lake District.     

Experimental Studies of Apospory in Ferns

Apospory in Pteridium aquilinum was found to have several novelfeatures, in particular the rapidity of its occurrence. Outgrowthsfrom detached laminae were detected in as little as 3 days.There was no evidence that failure of co-ordination betweencells, senescence of the sporophytic tissue or unfavourableculture conditions effected the initiation of outgrowths asearlier workers have suggested. The response of Thelypterispalustris was notably slower and more in line with earlier reportsof apospory. Marsilea vestita appeared to be incapable of anyform of aposporous behaviour. The only feature common to all occurrences of apospory in leavesof wild-type ferns is the severance of the lamina from the mainbody of the plant. It is proposed that this may deprive thecells of regulatory substances which maintain the activationof sporophytic genes. The results are also considered in relationto the phase changes in homosporous and heterosporous plantsand the behaviour of fern protoplasts in culture. apospory, ferns, Pteridium aquilinum, Marsilea vestita, Thelypteris palustris