Ecological Implications of Azolla sp. Proliferation Events in the Lower Broken Creek

"May 2006".

Project Number: Ecological model and data assessment, Azolla - M/BUS/69-2.

MDFRC item.

34 pages.

The Broken Creek runs from the Broken River to the Murray River. Historically, the creek was ephemeral, with water derived from natural catchment flows. The construction of a series of small weirs and changes to flow regulations have drastically altered flows in the creek, with the lower section now comprising a series of weir pools. More recently, the original structures have been replaced with modified structure that include fish passages. Broken Creek now display a series of undesirable characteristics. Notably, extensive growth of the floating fern Azolla occurs over large sections of the creek, for reasonably long periods of time. A fish death incident occurred in Rices Weir (the last weir on the creek) in 2002 and the Azolla growth was implicated in contributing to the poor water quality in the creek. This purpose of this report was to review the current literature on the growth and ecology of Azolla, to assess the current condition of the Broken Creek (including factors that lead to Azolla growth), to develop a conceptual model describing the factors that may be leading to the outbreaks and decline of Azolla and to make management recommendations for the potential control of Azolla. The report also highlighted knowledge gaps that prevent clear understanding of the ecological processes in the creek. The report concentrates on the ecological condition of Rices Weir as this represents the “worst case” scenario for the creek and that the successes of any intervention would ultimately be gauged against this weir pool. The literature review established that a large proportion of the information on Azolla has been carried out from the perspective of the “green manure industry”, where Azolla is grown in parallel with rice crops as a nutrient source. Less information on Azolla’s ecological significance exists. Be that as it may, importance characteristics have been established on Azolla’s growth form, mode of reproduction, temperature requirements and nutrient requirements to allow this report to establish reasons for the growth bloom. This report could not unequivocally establish reasons for the decline in the Azolla. Present operation of the creek has lead to conditions highly conducive to Azolla growth. During peak periods of growth, the creek contains warm and very slow flowing water with very high nutrient levels (suitable for growth of Azolla). Longterm nutrient inputs, sedimentation and recycle of dead Azolla is likely to have given rise to large carbon and nutrient pools in the sediments. Anaerobic conditions in the sediments will lead to release of nitrogen and phosphorus into the water column. Furthermore, sulfate levels in the Broken Creek are sufficient for sulfate reduction to occur in the sediments, indicating that the sulfur cycle (microbial processes) may be having an important role in the water chemistry.

A unequivocal explanation for the decline of Azolla is lacking in the conceptual model developed in this report. Lethal temperatures do not appear to occur in the creek and nutrients generally remain high. However, the report suggest that the large accumulations of Azolla eventually gives rise to nutrient and light limitation with the large masses, that the doubling time of the plants declines to a point where the discharge is sufficient to remove plants. Some plants certainly die in situ, leading to some recycle of carbon and nutrients to the sediments. Dissolved oxygen (DO) levels remain a key aspect of water quality in the Broken Creek. The report established that a series of factors lead to the present DO. DO at any given point is greatly affected by water quality from upstream as well as the extent of sediment processes and that Azolla surface coverings exacerbate the problems of low DO. An overall assessment is that water quality in the lower sections of the creek is very poor and that the long term operation of the creek has given rise to the present problems. Short term solutions are not likely, but longer term improvements may be possible. For any improvements in water quality to occur, the following management will be required: Removal of Azolla. Either through mechanical (although costly) or increased flow. For the latter, downstream effects of water quality on the Murray River will need to be assessed. As well as improving water quality, removal will have long-term effects on sediment carbon and nutrient loads. The success of removal strategies is in part speculative as the scale of the problem is large and removal activities have not been carried out at this scale elsewhere; Nutrient control at catchment scale. Control through managing nutrients is not likely to reduce Azolla growth in the short-term as indications are that very high levels of nutrients occur in the sediments. Nutrient management, through improved catchment condition is more likely to have benefits over the longterm and should be considered as part of a long-term management plan; Flow manipulation. The present flow does appear to have a role in reducing the effects of Azolla on water quality, although the present flows themselves alone are not effective in flushing Azolla from the system. Flow manipulation would seem to be the best option for dealing with what can only be seen as a “very difficult” problem to solve, but its role is to prevent excessive accumulation of Azolla in the waterway rather than reducing the rate at which Azolla grows. In essence, the present conditions are maintained for extractive water use resulting in low to zero flow periods with a consequence being the development of Azolla blooms.

Monitoring. Temperature and oxygen monitoring are providing valuable information on water quality. Further examination of the degree of stratification that occurs within Rices Weir should be carried out to gain better insight to the overall water quality in the Creek. Better estimation of the Azolla biomass should be made as the current visual estimate of percentage cover gives inconclusive estimates of the amount of plant present. Nutrient analyses should occur in concert with measures of Azolla biomass, so that declines in Azolla populations can be predicted. Knowledge gaps are summarised under three general categories; Sediment chemistry. The sediments in the lower Broken Creek are likely to be a major source of carbon and therefore are likely to have significant oxygen demand, thus affecting the overall water quality. Furthermore, it is likely that the sediments also are a major pool of nutrients, especially phosphorus and nitrogen. Measures of sediment oxygen demand and nutrient composition should be carried out so that long-term estimates can be made of the capacity of the sediments to deliver nutrients to the water column. Sediment analyses will establish whether nitrogen, phosphorus and products of sulfate-reduction are present in sufficient quantities and appropriate form to have an important role in the nutrient status of the weirs. Sediment analyses also will help establish if there is a possible point source responsible for the low DO or whether the DO decreases are “driven” by carbon and nutrients within the sediments; Nitrogen and Phosphorus dynamics. The links between different N species, P and Azolla growth is not known. Since nutrients are generally high in the Broken Creek, phosphorus control may remain the only nutrient-management strategy that is likely to have any long-term effect. Questions include, under what conditions does Azolla become N-limited (no longer uses inorganic nitrogen) and nitrogen fixation becomes a major source of nitrogen? What effect does this have on growth dynamics (especially doubling times and biomass) of Azolla?, If anoxic conditions occur at depth in the weirs, does denitrification occur, thus actually removing excess nitrogen from the system?; Azolla growth dynamics. Azolla biomass estimates have been made at irregular intervals and simply based on subjective visual estimates of the percentage cover. Quantitative measures of the Azolla biomass should be considered in the future, but will require reasonably rigorous sampling over a wide spatial and temporal range. Initial effort could be expended within Rices Weir, which would establish factors in the “worst case scenario”, but measures also would need to be carried out up stream. What is limiting growth in the lower Broken Creek? The role of nutrient supply and factors leading to the decline are testable hypotheses. Ideally, the best approach would be to track an Azolla bloom, measuring biomass, nutrients, oxygen, discharge and upstream inputs. This approach would allow a better understanding of the factors that lead to boom and bust of Azolla.