Water quality as a regional driver of coral biodiversity and macroalgae on the Great Barrier Reef.

Degradation of inshore coral reefs due to poor water quality is a major issue, yet it has proved difficult to demonstrate this linkage at other than local scales. This study modeled the relationships between large-scale data on water clarity and chlorophyll and four measures of reef status along the whole Great Barrier Reef, Australia (GBR; 12-24 degrees S). Four biotic groups with different trophic requirements, namely, the cover of macroalgae and the taxonomic richness of hard corals and phototrophic and heterotrophic octocorals, were predicted from water quality and spatial location. Water clarity and chlorophyll showed strong spatial patterns, with water clarity increasing more than threefold from inshore to offshore waters and chlorophyll decreasing approximately twofold from inshore to offshore and approximately twofold from south to north. Richness of hard corals and phototrophic octocorals declined with increasing turbidity and chlorophyll, whereas macroalgae and the richness of heterotrophic octocorals increased. Macroalgal cover experienced the largest water quality effects, increasing fivefold with decreasing water clarity and 1.4-fold with increasing chlorophyll. For each of the four biota, -45% of variation was predictable, with water quality effects accounting for 18-46% of that variation and spatial effects accounting for the remainder. Effects were consistent with the trophic requirements of the biota, suggesting that both macroalgal cover and coral biodiversity are partially controlled by energy supply limitation. Throughout the GBR, mean annual values of >10 m Secchi disk depth (a measure of water clarity) and < 0.45 g/L chlorophyll were associated with low macroalgal cover and high coral richness, indicating these values to be potentially useful water quality guidelines. The models predict that on the 22.8% of GBR reefs where guideline values are currently exceeded, water quality improvement, e.g., by minimizing agricultural runoff, should reduce macroalgal cover on average by 39% and increase the richness of hard corals and phototrophic octocorals on average by 16% and 33%, respectively (all else being equal). Such guidelines may help focus efforts to implement effective pollution reduction and integrated coastal management policies for the GBR and other Indo-Pacific coral reefs.

[1]  J. Friedman.  Greedy function approximation: A gradient boosting machine. , 2001 .

[2]  R. Steneck,et al.  Interaction between inorganic nutrients and organic matter in controlling coral reef communities in Glovers Reef Belize. , 2005, Marine pollution bulletin.

[3]  J. Friedman.  Stochastic gradient boosting , 2002 .

[4]  Jonathan M. Chase,et al.  Spatial scale dictates the productivity–biodiversity relationship , 2002, Nature.

[5]  M. Littler,et al.  Assessment of coral reefs using herbivory/nutrient assays and indicator groups of benthic primary producers: a critical synthesis, proposed protocols, and critique of management strategies , 2007 .

[6]  D. Bellwood,et al.  Confronting the coral reef crisis , 2004, Nature.

[7]  D. Burkepile,et al.  Herbivore vs. nutrient control of marine primary producers: context-dependent effects. , 2006, Ecology.

[8]  G. De’ath,et al.  PHOTOSYNTHETIC SYMBIONTS AND ENERGY SUPPLY DETERMINE OCTOCORAL BIODIVERSITY IN CORAL REEFS. , 2008, Ecology.

[9]  M. Hay,et al.  Effects of nutrients versus herbivores on reef algae: A new method for manipulating nutrients on coral reefs , 1999 .

[10]  T. Hardy,et al.  Circulation in the Great Barrier Reef Lagoon using numerical tracers and in situ data , 2007 .

[11]  B. Lapointe,et al.  The relative importance of nutrient enrichment and herbivory on macroalgal communities near Norman's Pond Cay, Exumas Cays, Bahamas: a "natural" enrichment experiment , 2004 .

[12]  K. Fabricius.  Effects of terrestrial runoff on the ecology of corals and coral reefs: review and synthesis. , 2005, Marine pollution bulletin.

[13]  Glenn De'ath,et al.  IDENTIFYING ECOLOGICAL CHANGE AND ITS CAUSES: A CASE STUDY ON CORAL REEFS , 2004 .

[14]  John J. Cullen,et al.  Sunlight and water transparency: cornerstones in coral research , 2002 .

[15]  Brian G. Wolff,et al.  Forecasting Agriculturally Driven Global Environmental Change , 2001, Science.

[16]  David Haynes,et al.  Assessment of the Water Quality and Ecosystem Health of the Great Barrier Reef (Australia): Conceptual Models , 2007, Environmental management.

[17]  B. Maxwell,et al.  Humans, Hydrology, and the Distribution of Inorganic Nutrient Loading to the Ocean , 2003 .

[18]  Y. Loya,et al.  Nutrient enrichment caused by in situ fish farms at Eilat, Red Sea is detrimental to coral reproduction. , 2004, Marine pollution bulletin.

[19]  O. Hoegh‐Guldberg,et al.  Kinetics of photoacclimation in corals , 2002, Oecologia.

[20]  C. Goldspink,et al.  Distributional patterns and community structure of Caribbean coral reef fishes within a river-impacted bay , 2007 .

[21]  G. De’ath,et al.  Species richness and community structure of reef-building corals on the nearshore Great Barrier Reef , 2006, Coral Reefs.

[22]  B. Schaffelke.  PARTICULATE ORGANIC MATTER AS AN ALTERNATIVE NUTRIENT SOURCE FOR TROPICAL SARGASSUM SPECIES (FUCALES, PHAEOPHYCEAE) , 1999 .

[23]  G. De’ath.  Boosted trees for ecological modeling and prediction. , 2007, Ecology.

[24]  Jon Brodie,et al.  Sources of sediment to the Great Barrier Reef World Heritage Area. , 2005, Marine pollution bulletin.

[25]  Britta Schaffelke,et al.  Short-term nutrient pulses as tools to assess responses of coral reef macroalgae to enhanced nutrient availability , 1999 .

[26]  W. Wiebe,et al.  Latitudinal limits of coral reef growth , 1983 .

[27]  Jon Brodie,et al.  Spatial and temporal patterns of near-surface chlorophyll a in the Great Barrier Reef lagoon , 2007 .

[28]  T. Hughes.  Community Structure and Diversity of Coral Reefs: The Role of History , 1989 .

[29]  Jon Brodie,et al.  Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence. , 2005, Marine pollution bulletin.

[30]  R. Fraser,et al.  The Species Richness-Energy Hypothesis in a System Where Historical Factors Are Thought to Prevail: Coral Reefs , 1996, The American Naturalist.

[31]  F. Rohwer,et al.  Indirect effects of algae on coral: algae-mediated, microbe-induced coral mortality. , 2006, Ecology letters.

[32]  G. De’ath,et al.  Changes in algal, coral and fish assemblages along water quality gradients on the inshore Great Barrier Reef. , 2005, Marine pollution bulletin.