Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment.

That major flows of energy occur along detrital pathways in all ecosystems is a recent recognition. In freshwater ecosystems, detritus or dead organic matter (217) has two possible sources: autochthonous detritus generated within the ecosystem and allochthonous detritus generated externally. This review is concerned with the breakdown of vascular plant detritus whether autochthonous, from aquatic vascular plants, or allochthonous, derived from riparian trees and herbs. The importance to the energetics of streams of vascular plant material from riparian vegetation was recognized in early studies by Nelson & Scott (184), Egglishaw (85), and Minshall (175). Organic matter budgets for various streams have provided quantitative data to support these early observations (96, 132, 182, 254). However, many low-order streams that lack canopies of riparian vegetation may be dominated by autochthonous primary production of nonvascular plant origin. (72, 176). Theoretical models (256) predict increasing importance of autochthonous production by periphyton and aquatic vascular plants for middle-order streams but less importance of these sources in very large streams, mainly due to light limitation. The relative dominance of allochthonous vs autochthonous sources has been shown to vary between stream systems and with local conditions within streams (72, 178). While vascular plant leaves have received most attention in stream research, there is growing recognition that wood is also important. The direct contribution of wood to stream energy budgets is minimal because wood is resistant to breakdown (e.g. 8, 251). However, woody debris is indirectly important because it creates habitat for aquatic organisms (5), promotes

Vascular plant breakdown in freshwater ecosystems

Stream ecologists are faced with the problem of recognizing patterns in community organization and explaining the processes that determine these patterns. I argue that streams con- form reasonably closely to the patch dynamics explanation of community organization which emphasizes temporal phenomena and focusses on the importance of history and chance. Even where competition or predation have been shown to play a role in shaping stream communities, the temporal phenomena of disturbance and colonization are invariably also of fundamental importance. In most cases, temporal variation is probably the factor of overriding significance, and species with weedy characteristics are a particularly prominent feature of streams. I highlight the critical role played by refugia as sources of recolonization after spates, and therefore as buffers against distur- bance. A theoretical framework based on the patch dynamics view of community ecology may provide a unifying theme in stream ecology.

The Patch Dynamics Concept of Stream Community Ecology

SUMMARY. 1. Macro-invertebrate species lists were obtained for 268 sites on forty-one river systems throughout Great Britain by qualitative sampling in spring, summer and autumn. Information on twenty-eight environmental variables was also collated for each site. The sites were ordinated on the basis of their species content using detrended correspondence analysis (DCA) and classified by two-way indicator species analysis (TWINSPAN). Correlation coefficients between ordination scores and single environmental variables indicated that Axis 1 distinguished between types of rivers and Axis 2 reflected variation along the length of rivers. A preliminary classification of sites into sixteen groups has been proposed, together with a key which allows new sites to be classified. Information on the species and environmental features which characterize each group is also presented.



2. Multiple discriminant analysis (MDA) was employed to predict the group membership of the 268 sites using the twenty-eight environmental variables. 76.1% of sites were classified correctly. An independent assessment of predictive ability using forty test sites yielded a 50% success rate. Predictive ability was higher for the classification presented in this paper than in fifteen additional classifications produced using data from single seasons and/or different taxonomic treatments.



3. TWINSPAN and MDA were found to be useful approaches to the classification of running-water sites by their macro-invertebrate fauna and the prediction of community type (as indicated by the occurrence of species in the sites comprising the group) using environmental variables. Extension of the scope of the classification, coupled with the use of additional environmental variables to increase predictive ability, is now in progress.

A preliminary classification of running‐water sites in Great Britain based on macro‐invertebrate species and the prediction of community type using environmental data

▪ Abstract We review the major conceptual developments that have occurred over the last 50 years concerning the factors that influence insect biodiversity in streams and examine how well empirical descriptions and theory match. Stream insects appear to respond to both spatial and temporal variation in physical heterogeneity. At all spatial scales, the data largely support the idea that physical complexity promotes biological richness, although exceptions to this relationship were found. These exceptions may be related to how we measure habitat complexity at finer spatial scales and to factors that influence regional richness, such as biogeographic history, at broader spatial scales. However, the degree to which local stream insect assemblages are influenced by regional processes is largely unknown.

Biodiversity of stream insects: variation at local, basin, and regional scales.

SUMMARY. 1. Invertebrates and fish were surveyed during October 1976 in thirty-four stony riffle stream sites in Ashdown Forest, Sussex.



2. A variety of physicochemical factors were also measured in an attempt to assess the importance of each in determining the distribution of species and the structure of communities.



3. Three analytical techniques—stepwise multiple regression analysis, ordination and community classification—revealed that the structure of these communities was strongly related to variation in stream pH. Acid sites had low numbers of individuals, low species richness and low equitabilities. Summer temperature and stream discharge also appeared to play significant roles. The pattern of catchment land use was shown to have an important bearing on stream pH.



4. In the most acid sites only collectors, shredders and predators occurred. In more basic sites the number of species in collector and predator categories increased and these were joined by grazer/scrapers and filter feeders.



5. A theoretical basis for explaining stream community structure is discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2427.1983.tb00011.x

Community structure in some southern English streams: the influence of physicochemical factors

SUMMARY. 1. Ceilulolytic decomposition, measured by loss of tensile strength in strips of cellulose test cloth, was estimated in thirty-four stream sites in experiments in summer, autumn and winter. 2. The results of multiple regression analyses showed that strength loss was most closely related to temperature in summer, when the model accounted for only 38% of the variation, and to stream pH in autumn and winter, when the models accounted for 52% and 75% of the variation, respectively. 3. The relationship between decomposition rate and the structure of invertebrate communities in streams of dissimilar pH is discussed. We argue that a very rapid rate of decomposition of coarse particulate organic matter may profit consumers of fine particulate organic matter at the expense of coarse particle shredders.

Cellulolytic decomposition in streams of contrasting pH and its relationship with invertebrate community structure

ALAN G. HILDREW, COLIN R. TOWNSEND* and JEANETTE FRANCISSchool of Biological Sciences. Queen Mary College. London, and*School of Biological Sciences, University of East Anglia. NorwichSUMMARY. I. The results of a survey of thirty-four stream sites, differingin pH and invertebrate species richness, indicated that the pool of locallyavailable, suitably adapted species was smaller in the acid streams. Thisplays a part in determining the general pattern of lower species richness atmore acid sites.2. Diversity of feeding categories increased with species richness,indicating that a greater range of food resources was available in the lessacid, more species-rich communities.3. The pattern of predation varied with pH and species richness. Thenumbers of large insect predators were lower in the less acid, morespecies-rich communities and this was correlated with the presence of fish.4. A detailed study of the guild of detritivorous stoneflies in four streamsdiffering in species richness provided evidence that density compensationoccurs, niche width decreases and niche overlap declines as speciesrichness increases,5. We discuss the roles that competition and predation play indetermining the structure and richness of stream invertebrate communi-ties.

Jeanette Francis

Papers

Community structure in some southern English streams: the influence of physicochemical factors

Cellulolytic decomposition in streams of contrasting pH and its relationship with invertebrate community structure

Community structure in some southern English streams: the influence of species interactions