Acroloxus President, Dr Michael Jackson, has studied many different aspects of shallow lake ecosystems and in particular the macroinvertebrate communities that inhabit the littoral margins of lakes.




Dr Jackson is the joint-leader of an international research team studying the effects of antifoulant paints in freshwater systems, in particular the effects of tributyltin on shallow lake ecosystems. These studies focus on the theory of Alternative Stable States and the work is being carried out in association with Dr Carl Sayer at the Environmental Change Research Centre (ECRC) at University College London, Dr Mike Waldock at the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) and Dr John Boyle at the University of Liverpool. This research began with his PhD (view PDF version) studies at The University of East Anglia in southern England studying The Role of Littoral Macroinvertebrates in the Management of the Shallow Lakes of The Norfolk Broads.


Dr Jackson has also worked on lake restoration with Dr Martin Perrow and the staff of ECON, based at the University of East Anglia, in a study of the feeding habits of Eurasian perch (Perca fluviatilis) in relation to the establishment of artificial refuges in Alderfen Broad in the Norfolk Broads, and on the effects of boardwalk construction on invertebrate communities in a wetland in Heron's Carr.












Alderfen Broad Artificial Refuge Experiment

Rationale:

In healthy shallow lakes there is commonly a profusion of aquatic life with a rich diversity of aquatic plants, macroinvertebrates, fish and waterbirds. The water is usually clear for most of the year and the efficient light penetration allows a thick 'forest' of underwater vegetation to grow and flourish - this has been termed the 'vegetation-dominated clear state' (Scheffer 1989) and it is this state in which most of the Norfolk broads were found at the turn of the nineteenth century. The broads are known to have harboured at least one-sixth of the macroinvertebrate fauna of the British Isles at this time (Jackson 1997). However, during the fifties and sixties the broads were exposed to exceptionally high levels of nutrients and other pollutants that caused major changes to occur in the biota of the lakes. The water became turbid and the submersed plants were lost, the fish and bird populations decreased in diversity and the majority of the macroinvertebrates disappeared - culminating in the so-called 'non-vegetated turbid state' (Scheffer 1989).

In the turbid state the fish community consists mainly of planktivores and benthivores (e.g. cyprinids) that selectively 'hoover-up' the larger herbivorous zooplankton resulting in a dominance of cyanobacteria, characterised by frequent phytoplankton blooms. Conversely, in the clear-water state, the fish community is more diverse with a variety of piscivorous species (e.g. percids) consuming mainly planktivores and thereby releasing large zooplankton from predation and allowing phytoplankton to be kept in check (Perrow and Jowitt 1999).

Research so far has resulted in a number of large-scale management prescriptions to try to restore some of the broads to their former diversity (Moss et al 1996). These have included major changes to sewage treatment works; suction dredging of polluted sediments and biomanipulation of fish populations. Together these restoration attempts have led to clear water being achieved in a number of broads that were formerly turbid. However, a return to the highly diverse communities of the past has yet to be realised and biomanipulation has mostly failed to sustain a diverse plant, macroinvertebrate or fish community (Perrow et al 1999).

Until recently, little attention has been paid to macroinvertebrates in these restoration efforts, but new evidence suggests that their role in lake eutrophication processes may have been seriously underestimated. There is now thought to be a commensual relationship between invertebrate grazers and submerged macrophytes - the plants acting as templates for the attachment of periphyton, thus indirectly providing an important food supply for many macroinvertebrates whose grazing activities in turn prevent the plants from being shaded out (Brönmark and Vermaat 1998). In addition, there are crucial predator-prey interactions that rely to some extent on the structural complexity of the habitat. Without the structure provided by the plants, the macroinvertebrates have nowhere to shelter from predators or wave action, to feed or to reproduce. Generalised predators, such as perch, need sufficient macroinvertebrate prey to enable them to grow fast enough to complete ontogenetic dietary changes (Diehl and Kornijów 1998).

If they do not pass through these ontogenetic 'bottlenecks' the perch remain stunted and may never become piscivorous (Persson and Greenberg 1990). Without piscivorous predators such as perch the recruitment of young cyprinids is left uncontrolled and leads back to high water turbidity. Lakes with healthy perch populations had often been reported to have high water clarity with stable diverse submerged plant communities and there was growing circumstantial evidence that there was a positive correlation between the abundance of littoral vegetation and the abundance of piscivorous perch (Persson et al. 1991, 1992). Trophic cascade theory (see Carpenter & Kitchell 1993) predicted that a range of size classes of piscivorous perch acted as an effective top-down control on cyprinid fish recruitment, the effects of which would cascade down the food chain to contribute to a reduction in phytoplankton biomass. In other words, the presence of a healthy macroinvertebrates community may act as a feedback mechanism to biomanipulation and help to stabilise the system.

One problem with lake restoration attempts to date has been that a suitable macrophyte structure for macroinvertebrate colonisation may not immediately follow from biomanipulation. By providing a temporary artificial refuge for macroinvertebrate husbandry this problem ought to be partly overcome.

The Artificial Refuge:
    



Results:

Zooplankton

Macroinvertebrates
Conclusions:

Installation of the artificial refuge into Alderfen Broad has been highly successful in terms of zooplankton colonisation and productivity in the initial year, with a total of 15 species and 3 taxa being recorded.

The perceived role of secondary, especially zooplankton, productivity within the littoral zone of shallow lakes has generally fallen behind that of the pelagic (Lim & Fernando 1978). The larger grazing Cladocerans, Daphnia spp, can have a significant effect on phytoplankton populations, but this often depends on the nature of the fish community and the presence of macrophyte cover (Stansfield et al 1997).

In shallow lakes with perch present, the littoral zone becomes a more important feature in restoration attempts, due to the feeding behaviour and eventual top-down, regulatory function of the perch.

Increasing the area of submerged macrophyte structure, before natural colonisation happens, is one way of reversing the mechanisms that allow phytoplankton to dominate. The zooplankton community associated with the refuge in this study offer positive feedback into a reversal of poor environmental conditions.

Further monitoring for several years is clearly required of zooplankton, macroinvertebrates and the perch population, if the validity of such relationships are to be adequately concluded.

Herons Carr, Norfolk, UK.


     An evaluation of the effects of the construction of a new
       boardwalk on the invertebrate fauna of Heron's Carr


           Summer 1999


Aims and Objectives:

The choice of sampling methodology and number of sampling locations was based upon the priority given to meeting a number of potential objectives. These objectives can be divided into two categories - those that were concerned with the evaluation of the site for management purposes and those that sought to help with the interpretation of the ecology of Heron's Carr for education purposes.

Evaluation:

1. To evaluate the invertebrate interest of the site by comparison to similar
habitats both locally and nationally.

2. To discover if the proposed route of the boardwalk endangered
the survival of any particularly rare or threatened species.

3. To gather the necessary data to enable advice to be given on
how best to design the path of the boardwalk through the carr and
on the design of the boardwalk itself in order to maximise the invertebrate
interest of the site.

4. To make a quantitative comparison of the invertebrate fauna along the route of the
boardwalk before and after its construction.

5. From an evaluation of the impact of the boardwalk, to make recommendations for future management of the carr, paths and other linear features in order to maintain and if necessary enhance the invertebrate interest.

Interpretation:

1. To investigate the interaction between the structure of the vegetation and the different invertebrate communities that live in different habitats across the vegetational succession from open water through tussock sedge and carr to birch/ash woodland.

2. To show how particular groups, such as molluscs and beetles, can be used to indicate the relative importance of the site.

3. To show the temporal succession of different species and groups.

4. To make a detailed study of the invertebrates inhabiting particular features of the habitat, such as the 'pulk holes' and tussock structures.

Summary and Conclusions:

  • Heron's Carr was found to be unexpectedly rich in invertebrate species. 242 species and 31 other taxa were recorded, but it was clear that the actual total might well have exceeded 1,500 species had all the habitats been sampled across the seasons and had all the groups been collected and identified.


  • A surprising find was the unusually high number of fenland species living with the carr, including rarities such as Vertigo moulinsiana.


  • The carr was sampled on two occasions, June and August, and there were considerable differences in both species-richness and abundance suggesting that many species were following patterns of occurrence linked to their particular life cycles.


  • The sampling locations closest to the broad had the highest abundance of individuals in June as well as the highest number of species in August. This was probably linked to the distribution of tussock sedges which were found to harbour many invertebrate species. The sedgebeds in the central area of the carr were also comparatively species-rich.


  • Aquatic macroinvertebrates along the littoral margin were far more numerous than in the littoral margin of the remainder of the broad and it was thought that this may have been on account of the very shallow water in Heron's Carr offering refuge from fish predation.


  • A number of water beetles, a nymph of one Notable/Nb dragonfly, Sympetrum sanguineum, and three species of large-bodied cladocerans recorded from the lake littoral indicated that the water chemistry of Barton Broad was not overly enriched in the region of Heron's Carr.


  • A number of unusual molluscs were recovered mainly from the central sedgebeds. These included Des Moulin's Whorl Snail, Vertigo moulinsiana, an RDB3 and BAP species and a number of other typically fenland species, such as Vertigo antivertigo and Zonitoides nitidus.


  • Among the spiders were three Notable/Nb species - Philodromus albidus, Tetragnatha pinicola and Theridiosoma gemmosum. The presence of many long-jawed spiders, Tetragnatha sp., was again indicative of fenland conditions.


  • One of the rarer woodlice, Ligidium hypnorum, was found frequently throughout the carr together with a number of other commoner species. A more thorough search of the tussocks in particular might have produced more species.


  • The long-legged flies or Dolichipodidae, proved to be a very interesting group and produced the rarest species of the study, Dolichopus laticola, an RDB1 species. This species had only previously been recorded at Ormesby Broad and in the Bure Marshes.


  • Difficulties with the use of the sweep net caused by the abundance of Succineid snails meant that craneflies and other delicate-winged insects could not be identified.


  • 14 species of hoverfly were found that were mainly attracted to the yellow water traps. Six of these hoverfly species were described as Local. As was expected the hoverflies seemed to particularly favour the sunny glades within the carr where trees had toppled over leaving gaps in the canopy.


  • Dragonflies and damselflies were neither caught nor observed in the study but they might have been flying high above the canopy or taking refuge higher up in the tree cover.


  • Small battery-operated moth traps, known as Heath traps, were used on several occasions from June to September and one large trap was set at the nearby cottages. A total of 85 species were caught in the course of the study including one Notable/Nb species, the Cream-bordered Green Pea.


  • The moth-trapping showed that the Heath traps were catching species mainly from the immediate locality and that most of the species were flying furthest away from the broad. The largest catch was collected on one of the stormiest nights of the year!


  • Two Notable/Nb beetles were caught, a rove beetle, Oxytelus fulvipes and a ground beetle, Trechus rubens. A number of Local species were also collected including the Irish Flea Beetle, Aphthona nonstriata, the ground beetle Carabus granulatus, the marsh beetle Cyphon padi, the ant-like beetle Euconnus hirticollis and the two rove beetles Stenus bifovealatus and S. guttula..


  • It was concluded that the effects of constructing a boardwalk through Heron's Carr would be unlikely to be detrimental to the invertebrate fauna provided that the work was carried out carefully and that sensitive areas were avoided.


  • The most sensitive locations in our opinion were the tussock sedges close to the broad and the sedgebeds in the central area.


  • Recommendations were made that in the course of construction and in its upkeep, the boardwalk should never threaten the structure and character of the carr, which should be left as intact as possible, and that certainly no heavy machinery should be used in the construction process


  • As far as future management of Heron's Carr was concerned we would recommend a policy of non-intervention except perhaps the occasional clearance of some tree cover (should this not occur naturally) to encourage the growth of sedgebeds and tussocks.


  • The results show that areas of open fen left to their own devices, and completely free from human intervention for a period of more than 50 years, need not suffer from depletion in invertebrate species-richness. In fact, the evidence from Heron's Carr suggested completely the opposite. These findings ought to have far-reaching implications with regards to future fen management strategies.


  • Finally, in view of the not inconsiderable effort that went into this study, it is hoped that the information collected will be disseminated to as wide an audience as possible and perhaps incorporated into interpretative literature that could be provided to visitors using the boardwalk.


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