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Technical Features
ALGAL BLOOMS, ESTUARINE HEALTH AND SUSTAINABLE COASTAL DEVELOPMENT IN AUSTRALIA Why a comprehensive policy document on eutrophication and bio-monitoring programs are urgently needed JA Phillips
ABSTRACT Recent scientific advances have emphasised that anthropogenic nutrient enrichment and algal biomass estimates are not good predictors of eutrophication, a serious problem affecting estuaries worldwide. Rather, an understanding of undesirable disturbance to the balance of organisms in the ecosystem provides a more accurate diagnosis. Marked shifts in algal species composition and abundance progressively occur with increasing nutrient enrichment and the consequent environmental degradation caused by large amounts of algal bloom-generated organic matter. These shifts, based on differing eco-physiological responses of algal species to nutrient enrichment, provide an effective tool for assessing risks and impacts of eutrophication. Estuarine monitoring programs are currently being developed in which the biotic indices derived from algal community structure are supported by physico-chemical indices. A comprehensive policy document on eutrophication, based on current rigorous science, and bio-monitoring programs are both urgently needed for assessments of estuarine health and the sustainability of coastal development.
ESTUARINE HEALTH
Keywords: algal blooms, estuarine health, sustainable development, eutrophication, phytoplankton, community structure.
INTRODUCTION Extensive coastal development driven by the rapidly expanding human population has already transformed large sections of the east Australian coastal zone (e.g. the Sydney metropolitan area and southern Queensland) into urban areas. This demographic trend, which will result
WATER SEPTEMBER 2013
in the inevitable loss or degradation of natural ecosystems, is expected to continue, with projections in the 2006 Australia State of the Environment report of the urbanisation of 42.3% of the coastal zone between Noosa and Nowra by 2050. Although urbanisation presents the strong potential for pollution of many different kinds, the most serious pollution problem impacting estuaries worldwide is excessive nutrient enrichment from anthropogenic sources. As naturally highly productive ecosystems, estuaries are particularly vulnerable to increased nutrient loading. These ‘nutrient sinks’ accumulate and retain nutrients by flocculating, adsorbing and sedimenting 90% of the nutrients in particulate organic matter during the mixing of fresh and salt water (Levin et al., 2001). Increased nutrient loading stimulates the growth of some phytoplankton and macro-algal species and potentially leads to the formation of algal blooms. Prolonged nutrient enrichment has fuelled decadal-long algal blooms that have significantly damaged many estuarine systems in Europe (e.g. the Baltic Sea), North America (Chesapeake Bay) and Australia (Peel Harvey Inlet, WA, McComb and Lukatelich, 1995; and the Gippsland Lakes, Victoria, Cook and Holland, 2012).
DEFINING EUTROPHICATION Until the 1990s, eutrophication was considered to be an enrichment process in which excessive nitrogen (N) and phosphorus (P) loading into aquatic ecosystems caused a marked decline in water quality. Estuarine monitoring programs have traditionally measured physical and chemical factors, with water column nutrient (particularly
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP)) levels and Chlorophyll-a (Chl-a) concentrations playing key roles in determining water quality. These two parameters are now known to be poor predictors of the risk of algal blooms, evident from studies of 51 estuaries where only 36% of the variance in phytoplankton Chl-a correlated to the N loading rate (Borum, 1996; Cloern, 2001). The simple nutrient loading-increased algal growth model fails to address the variability in, and complex responses to, nutrient-loading characteristic of estuarine ecosystems. Sampling frequency (e.g. once a month) providing periodic ‘snapshot’ measures of DIN and DIP may fail to detect the temporal and spatial complexity of nutrient availability in estuaries, where nutrients are often delivered in pulses and during non-sampling periods. Many phytoplankton and macroalgal species experiencing these varying nutrient levels often exhibit an asynchrony between external nutrient supply and growth rate. They avoid nutrient limitation by absorbing and storing nutrients during high ambient nutrient conditions for future growth during favourable (often seasonal) environmental conditions (Fujita, 1985; Smayda, 1997b). Furthermore, bloom species exhibit rapid nutrient uptake rates and tight nutrient cycling, often resulting in low water-column nutrient levels surrounding the bloom (Lapointe and O’Connell, 1989; Schindler and Vallentyne, 2008). Chl-a concentration may be low when nutrient concentrations are high due to light limitation of photosynthesis owing to turbid water and/or short water residence times, which quickly remove the algal biomass from the estuary. As well, Chl-a