Water Framework Directive Reference Conditions and Eutrophication Impacts in Irish Rivers

ERTDI Report 59 - A. Walsh

Summary: Synthesis Report of ERTDI-funded project: 2000-FS-2-M1

Published: 2007

ISBN: 1-84095-223-7

Pages: 13

Filesize: 106KB

Format: pdf

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Executive Summary

An important objective of this project was to obtain an improved understanding of why ecological communities depart from reference conditions as pollution and eutrophication impact on individual species and in particular indicator taxa such as Ecdyonurus. The effects of eutrophication on Ecdyonurus were studied using a novel split-stream experiment which involved artificially increasing the phosphorus (P) concentrations in two oligotrophic rivers in the West of Ireland. Some of the nutrient manipulation experiments showed significant differences in algal biomass between the control and treated sections, but not all did so.

 

The experiments did reveal surprising results showing the importance of nitrogen (N) limitation in the rivers studied. On analysing the N/P ratios in a number of rivers in the West of Ireland it was found that approximately 4% of the samples were N-limited with low molybdate-reactive P (MRP) concentrations (<0.05 mg/l P). This implies that a small proportion of high-status rivers are N-limited rather than P-limited. Thus, in terms of the Water Framework Directive (WFD), there may be a case for introducing tighter regulations on the limits of nitrogen emitted to waterbodies as well as the need to control phosphorus. Results from these studies highlight the complexity of the in-stream processes driving these N-limited and P-limited high-status rivers.

 

It was hypothesised that the changes exhibited in the biomass in the split-stream experiment would be reflected in the gut contents of Ecdyonurus venosus, possibly showing a variation in algal taxa between both sections of the manipulation experiment. The split-stream experiment showed that Ecdyonurus did not demonstrate distinct preferences due to enrichment but neither were there any observed changes in the periphyton species community. On the basis of the findings from the study in the Castlebar River in 2003, E. venosus can be classified as both a herbivorous grazer and detritus feeder with a tendency towards opportunistic feeding. The food preference of these larvae appears to be strongly dependent on the food available in the environment at a given time and they seem to feed on particles that are most abundant during a particular season or those that are easily accessible during a feeding episode. The Ecdyonurus gut contents consisted mainly of epilithic algal tissue, plant particulate matter (detritus), biofilm matrix and inorganic debris (mineral material).

 

The life cycle of the Heptegeniidae was described for the first time in detail in five high-status rivers in the West of Ireland. Of the three species of Ecdyonurus studied, E. venosus was the dominant species in all five highstatus rivers displaying a bivoltine life cycle with only a slight variation in emergence periods between sites. Findings show that one would expect to find this species when sampling during all seasons throughout the year. The life cycles of both E. insignis and E. dispar were found to be univoltine. The life-cycle analyses in this study suggest that at least one species of Ecdyonurus should be present at all times of the year. The life cycle of Rhithrogena semicolorata was substantially easier to interpret and clearly displayed a univoltine life cycle. The Heptagenia specimens were identified to genus level only and were therefore described as a genus group that appeared to adopt a univoltine life cycle. Findings from our studies support the hypothesis put forward that the various species of Ecdyonurus emerge in overlapping phases such that during the summer months larvae of at least one Ecdyonurus species will be present in the benthic riffle fauna of Irish rivers.

 

Ecdyonurus is a good indicator of pollution and the water chemistry results appear to support the hypothesis that the presence of Ecdyonurus is associated with good water quality. The presence/absence of Ecdyonurus in the high-status versus impacted sites supports its use as a significant bioindicator of water quality. A selection of indices and metrics were examined and the results revealed that the most significant differences between the high-status and impacted sites were found using Margalef’s index, total number of taxa and percentage Ephemeroptera/Plecoptera/Trichoptera (%EPT). Results from feeding and microhabitat investigations suggest that as eutrophication and the impacts of organic pollution progress, a change occurs in the feeding guilds and microhabitat preferences among the macroinvertebrate communities.