Eutrophication is the situations where nutrient enrichment, increased algal growth and/or increased organic production rates have resulted in change in benthic community structure. This definition is derived from Bell et al. (2007) and international eutrophication assessments (Foden et. Al 2010).
This has been a problem since the first European settlers arrived in 1850s and started expanding their agricultural practices, increasing the discharge in water of contaminants. Nowadays, an increase in the fertility of the sediments and water column of the Great Barrier Reef shouldn’t surprise us, if we consider the great amount of deforestation and agricultural development along the coast of Queensland.
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According to the Department of Biological Science and Centre for Marine Science, University of North Carolina at Wilmington, nutrient enrichment should be considered a major cause of coral decline.
In particular the elevated quantities of chlorophyll a along intensive and extensive phytoplankton blooms suggest that the Great Barrier Reef is significantly influenced by nutrients overload.
But how is eutrophication affecting the Great Barrier Reef?
The growth of benthic algae, heterotrophs (such as bacteria and viruses) and phytoplankton will be promoted by the increase of nutrients to a healthy coral reef region; this great growth might cause significant changes in the coral reef community structure (e.g. Bell 1992; Littler et al. 2009).
For example, the competition between phytoplankton and the zooxanthellae, microorganisms that live in symbiosis with corals, for light can interfere with coral growth.
Moreover, calcification rates and increase coral bleaching can happen as a consequence of stimulated growth of the zooxanthellae.
Benthic algae, also, can be threatening in many ways: they trap sediments, compete directly with corals for space, weaken the coral structure by entering the coral matrix and alone, or together with a variety of heterotrophs, promote CSDs, which were proven to be related to eutrophication.
Lastly, the conclusion that the nutrient pool of the Great Barrier Reef had reached a critical level for its survival some decades ago (Bell 1991, 1992; Bell and Gabric 1990, 1991; Bell and Elmetri 1995) is supported by the many applications of the ETM to the water quality and ecological data of this area and by the evident replacement of hermatypic corals with other benthos in these regions.
Thanks to the high ambient light intensities and water temperatures, available nutrients are easily converted to organic matter by phytoplankton, particularly in the inter-reef regions (Furnas er al. 2005).This large quantity of nutrients determines the water quality status and the impacts of benthic organisms.
Measurements of phytoplankton biomass as chlorophyll a are 2-3 times higher in inshore waters of the central and southern Great Barrier Reef (0.3-0.7 mg 1−1 ) compared to the northern areas (o.2 mg 1-1) (Brodie et al. 2007); these values are believed to reflect nutrient enrichment, associated with eutrophication caused by coastal human activities.
Moreover, the discharge of land sourced nitrogen and phosphorus flux in the Great Barrier Reef cause than increase in the extensive phytoplankton blooms (Brodie and Mitchell 2005, 2006); these changes in phytoplankton population have often been observed in different marine environments which were exposed to anthropogenic eutrophication.
The results of Bell and Elmetri (1995) also shows that there would be an increase in the production of smaller secondary producers, because of a great decrease in the diatom-flagellate ratio, and this could destabilize the food chain.
Furthermore, there is scientific evidence that the growth of dangerous corallivore such as COTS (and probably Drupella spp.) will be encouraged by changes in phytoplankton class structure.
As far as the problem of chlorophyll a is concerned, evaluations of time-series and spatial data from Kaneohe Bay (Hawaii) and Barbados were used to create the Eutrophication Threshold Model (ETM) for coral reefs. Further studies and applications of the ETM suggested that ETC-Chl a (~0.2–0.3 mg m−3) is an alarming value in regions where settlement of POM and a build-up of DOM are promoted, as well as in regions with many coral species easily affected by POMs.
The Great Barrier Reef has been severely damaged over the past 50 years by an increase in growth of coarallivores, such as COTS. This is also proved by large scale-monitoring data on the impact of COTS on the Great Barrier Reef (Sweatman et al. 2008; Osborne et al. 2011) and that circa 42% of coral loss in the Great Barrier Reef since 1985 is caused by them (De’ath et al. 2012).
It has been proved by findings that COTS larval growth is encouraged in the lower ETC-Chl a range 0.2–0.3 mg m−3, therefore showing that a chronic state of eutrophication would be defined more accurately at a range of >0.2 mg m−3 and that the proliferation of COTS in directly caused by the degree of eutrophication.
Furthermore, significant damage has occurred to the Great Barrier Reef over the past 50 years due to the proliferation of corallivores (e.g., COTS). Large-scale monitoring data show that COTS have impacted most regions of the GBR (Sweatman et al. 2008; Osborne et al. 2011); it is estimated that 42 % of all coral loss in the GBR since 1985 is due to COTS (De’ath et al. 2012). As said before, there is experimental evidence that demonstrates the critical Chl a concentration for survival and growth COTS larvae is within the lower ETC-Chl a range ~0.2–0.3 mg m−3 and thus supports the hypothesis that the outbreaks of COTS can be linked directly to the degree of eutrophication.
Figure 1, comparison of cross-shelf variation of chlorophyll a data (R&G 17, 35; F&M, 49) in Central GBR lagoon with suggested eutrophication threshold values .
Figure 2, summary of long-term GBR monitoring data (AIMS 2012); the image shows regions of chronic eutrophication by annual mean chlorophyll a values >0.2 mg m−3
As showed by long-term monitoring by the Australian Institute of Marine Science, the quantity of hermatypic corals in the Great Barrier Reef has decreased by ~51 % since 1985.
The principal causes of this are the outbreak of corallivores, like the crown-of-thorns starfish and COTS, and coral skeletal diseases; this has been proved to be a consequence of anthropogenic development, and in particular to eutrophication (GBRMPA 2010; Brodie and Waterhouse 2012 ;Kuta and Richardson 2002; Aeby et al. 2011; Haapkyla et al. 2011).
This theory was also proved by controlled laboratory and field-based studies, which showed that adding nutrients increased the rate of growth of COTS and host tissue loss loss (Bruno et al. 2003; Voss and Richardson 2006).
Moreover, as Voss and Richardson (2006) proved that small increases in N and P concentrations to values around the Nutrient Threshold Concentrations (NTCs; Bell 1992; Bell et al. 2007) increased the rate of expansion of BBD.
The quantity of hard corals in the GBR region has reduced by >70 % since development of the coastal catchments. The principal causes of their loss are attributed to the widespread growth of COTS and CSDs, and it is now widely accepted that this is attributable to eutrophication.
Much of the increased eutrophication is caused by the increased loads of nutrients discharged from coastal developments, especially phosphorus, nitrogen and chlorophyll a.
Authorities have recently taken significant action aimed at reducing runoff nutrient loads. However, further action is required to minimize the impacts of point-source discharges and particularly of P-PO4 rich discharges.
Also further investigations on the links between eutrophication and the proliferation of CSDs and coral bleaching need to be conducted.
Some reefs in regions characterized by annual mean Chl a concentrations in the lower range of the proposed ETCs namely ETC-Chl a ~0.2–0.3 mg m−3 show good resistance to physical damage but the available evidence suggests that CSDs and COTS will proliferate in such waters and therefore the eutrophication trigger values for Chl a will need to be decreased to ~0.2 mg m−3 for sustaining coral reef communities.
Moreover, nutrient enrichment will stimulate the growth of phytoplankton, benthic algae and heterotrophs; this exponential growth can change significantly the coral reef community structure. The addition of nutrients to a healthy coral reef region will stimulate the growth of phytoplankton, benthic algae and heterotrophs (e.g., bacteria, viruses); this excessive growth can cause significant changes in the coral reef community structure.
Furthermore, the increased growth of the zooxanthellae can cause a decrease in calcification rates and increase coral bleaching.
Benthic algae, also, can be threatening in many ways: they trap sediments, compete directly with corals for space, weaken the coral structure by entering the coral matrix and alone, or together with a variety of heterotrophs, promote CSDs, which were proven to be related to eutrophication.
However, according to the Department of Biological Science and Centre for Marine Science, University of North Carolina at Wilmington over-enrichment cannot be cause of a widespread coral reef degradation, because other factors can cause significant damage to corals.
Also, most of the evidence came from laboratory studies, in which corals were exposed to high levels of nutrients for short periods. A limitation of these studies is the high nutrient concentration used to get results in periods of few weeks or a month: e.g. 20 to 200 mg Nitrate. These concentrations are much greater than the highest levels measured on polluted coral reefs.
Moreover, most of the experiments were not designed with the purpose of analyzing the effects of eutrophication on corals; therefore, the results must be interpreted with care.
It is also needed further research on the validity of the pollutants that are actually being targeted, but trying to lower the quantity of chlorophyll a is certain to reduce eutrophication in the Great Barrier Reef.
Lastly, a great limitation in detecting improvements in practices and measurable results in the Great Barrier Reef health is detecting the effect of time lags and the signal of change in the system.
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