Coral bioindicators of environmental conditions on coastal coral reefs

Cooper, Timothy F. (2008) Coral bioindicators of environmental conditions on coastal coral reefs. PhD thesis, James Cook University.

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Abstract

Reversing the decline in water quality is a key priority for the protection of the Great Barrier Reef (GBR). Strategies to improve the water quality of the GBR include conservation of riparian zones and the adoption of ecologically sustainable practices in the catchments. The implementation of these strategies requires feedback to resource managers and the community through monitoring programmes aimed at detecting biological responses to changes in water quality. This thesis investigates a range of coral indicators at different spatial and temporal scales and identifies those most suitable for inclusion into a toolbox for monitoring the condition of coastal coral reefs on the GBR. The approach combines in situ studies of coral indicators in different regions and environmental gradients on the GBR with controlled manipulative experiments exposing corals to differing water quality to examine causality of correlations observed in the field.

An environmental gradient was identified in the Whitsunday Islands where water column variables (especially chlorophyll a, total suspended solids, particulate organic carbon and particulate nutrients) and irradiance variables (Secchi and optical depth) differed significantly from nearshore to the outer islands. For example, mean concentrations of chlorophyll a were up to 1.9 times greater at nearshore (Repulse Island; RI: 0.59 ± 0.12 μg L-1 mean ± SE) compared with outer islands (Edward Island; EI: 0.31 ± 0.06 µg L-1) averaged over five sampling events from 2004 to 2006, whereas mean Secchi depth was approximately 3 times lower at nearshore (RI: 4.0 ± 0.8 m) than outer locations (EI: 15.3 ± 3.3 m). Some of the coral indicators showed significant relationships with a water quality index (WQI) derived for the Whitsunday Islands. Responses of photo-physiological measures of Symbiodinium associated with Pocillopora damicornis along the gradient were consistent with patterns of light acclimatisation and suggested deep corals (i.e. below 5 – 6 m depth) on nearshore reefs in the Whitsunday Islands are light-limited. Both colony brightness and tissue thickness of massive Porites spp., and the maximum depth of reef building corals, increased from nearshore to outer locations along the gradient. Similarly, a 50-fold decrease in the density of macro-bioeroders in massive Porites from nearshore to outer locations was indicative of increased particle loads on the nearshore reefs. The data of the maximum depth limit for coral reef development at locations where suitable settlement substrata were available suggest that the absolute minimum of light required for a coral reef to persist is in the range of 6 – 8 % of surface irradiance in the Whitsunday Islands.

The model that color brightness of corals responded to changes in water quality was validated with manipulative experiments in the laboratory and by transplantation of small nubbins along the environmental gradient. The experiments showed nubbins of massive Porites became darker, i.e. concentrations of pigments increased, within 20 – 40 days of exposure to elevated nutrients and reduced irradiances compared with corals kept in filtered sea water and unshaded conditions. The response in colony brightness was consistent with other studies of photo-acclimatisation to enhanced nutrients and light limitation. However, a 2.5-fold decrease in symbiont density of P. damicornis during the wet compared with the dry season, which in turn influenced colony brightness, was related strongly to seasonal changes in sea surface temperature (SST). Thus, effects of seasonal variation of a range of environmental parameters need to be considered if physiological measures such as colony brightness are used in water quality monitoring programmes.

The simultaneous in situ measurement of benthic irradiance and turbidity at a shallow depth (~3.5 m) on a coastal coral reef for 2 years allowed the quantification of potential thresholds of concern for turbidity. The linear relationship between the attenuation coefficient for downward irradiance Kd (PAR) and turbidity showed that a change from 0 to 3 nephelometric turbidity units (NTU) at 3.5 m results in a decrease of 88% of benthic irradiance to levels around 200 µmol photons m-2 s-1. The minimum saturating irradiance (Ek) of Symbiodinium associated with P. damicornis was approximately 206 ± 8 µmol photons m-2 s-1 at shallow depths on nearshore reefs of the Whitsunday Islands. Thus, levels of turbidity greater than 3 NTU can result in environmental conditions that are light limiting, and hence sublethal photo-physiological stress, for P. damicornis. Levels of turbidity of 4.5 NTU corresponded to 6 – 8% of surface irradiance, which was a critical level of irradiance required for coral reef development in the Whitsunday Islands. Thus, long-term turbidity >3 NTU could be used as a threshold of turbidity for sublethal photo-physiological stress, while long-term turbidity >5 NTU for severe stress effects on P. damicornis at shallow depths (~3.5 m) on coastal reefs.

Temporal variation in the growth parameters of massive Porites from two nearshore regions of the GBR were not consistent with regional differences in water quality. Mean annual SST increased by ~0.38°C over a 16 year study period that correlated with a decline of ~21% in coral calcification rates. A decline in calcification of this magnitude with increasing SST contrasts with results of previous studies and is unprecedented in recent centuries. Changes in the growth parameters were linear over time, while SST had no effect on skeletal density, but a modal effect on annual extension and calcification with maxima at ~26.7°C. The findings were consistent with other experimental studies of the synergistic effect of elevated seawater temperatures and CO2 partial pressure (pCO2) on coral calcification and suggest that monitoring of seawater chemistry should be undertaken on the GBR.

Defining a set of key selection criteria and assessing the characteristics of candidate indicators in a matrix against changes in water quality, allowed the identification of coral indicators for a monitoring toolbox. The most suitable bioindicators were: symbiont photo-physiology, colony brightness, skeletal and tissue growth, and bioeroder density in massive Porites, coral recruitment, community structure of corals, indicator organisms other than corals and the maximum depth of coral reef development. As each of these measures has a different sensitivity and response time to changes in environmental conditions, a combination of measures, i.e. a composite indicator system, is recommended for use in assessments of the condition of coastal reefs on the GBR.

Item ID: 8170
Item Type: Thesis (PhD)
Keywords: Great Barrier Reef, GBR, Whitsunday Islands, water quality, monitoring toolbox, coastal coral reefs, bioindicators, massive Porites spp.
Date Deposited: 05 Feb 2010 02:50
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960506 Ecosystem Assessment and Management of Fresh, Ground and Surface Water Environments @ 50%
96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 50%
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