Drivers of change in seagrasses of the northern Great Barrier Reef

Seagrass meadow at Magnetic Island
Seagrass meadow at Magnetic Island

Seagrass meadow at Magnetic Island

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The aims of this work (MTSRF Project 1.1.3b) were to identify the role of light and water temperature as drivers of change in seagrass meadows of the northern Great Barrier Reef. Experimental approaches as well as field investigations were undertaken.

Field studies occurred at four locations (Magnetic Island, Dunk Island, Green Island, Low Isles). There was continuous monitoring of light and temperature at seagrass canopy height in both intertidal (above lowest astronomical tide, LAT) and near-by subtidal meadows (1-3m below LAT) at each of the four locations.  Approximately every 3 months, seagrass response variables were also measured, including percent cover, growth, seagrass morphology and physiology.

There were three experiments testing the effects of temperature and/or light on tropical seagrasses.

  1. A shading experiment where seagrass response variables were measured for 102 days at four light levels: high light (66% surface light), moderate (31%), low (14%) and very low light (1%).
  2. A pulsed temperature experiment where short-term spikes of water temperature to 35, 40 and 43°C, which mimic current temperature ranges during low tide in the GBR, were applied for 6 days followed by one day of recovery.
  3. A temperature and light interaction experiment at 27, 30 and 33°C, which spans current and future predicted summer water temperature, at saturating (400 µmol photons m-2 s-1) and limiting light levels (40 µmol photons m-2 s-1).

Results

Field investigations

Seagrasses in intertidal meadows (above LAT) received much higher light levels than subtidal meadows (below LAT) and this corresponded to higher percent cover and meadow productivity rates in intertidal meadows. Of the subtidal meadows, Green Island had the highest average light and highest seagrass cover and diversity while Low Isles and Magnetic Island had the lowest light levels. At Magnetic Island, large declines in seagrass cover (45% cover down to 1% cover from January 2008 to April 2010) were associated with two successive monsoon seasons when light fell below the minimum light requirements for the dominant species present together with gradual deterioration in light levels throughout the monitoring period. Low Isles also had low average light levels, low seagrass cover and declining cover during and following the monsoon season of 2008/2009. There was no evidence of high temperature-related stress at any of the study sites.

Decline of seagrass meadow at Magnetic Island
Decline of seagrass meadow at Magnetic Island

Fig. 1: Left: The seagrass meadow at Magnetic Island in January 2008. Right: The same meadow in April 2010, after more than two years of declining light and three prolonged wet seasons.

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Aquaria experiments

  1. Shading experiments were run for 102 days at four light levels: high light (66% surface light), moderate (31%), low (14%) and very low light (1%). Four species were investigated: Cymodocea serrulata,Halodule uninervis, Thaalssia hemprichii and Zostera muelleri. All species responded to the low and very low light treatments in the following sequence: metabolic and physiological changes (reduced growth, increased pigment concentrations and photosynthetic efficiency); shedding (leaf loss, followed by shoot loss); and production of new, altered tissue (leaves with different dimensions including leaf length, width and thickness). Complete shoot loss was projected after 130 days. Zostera muelleri showed the fastest response and with the greatest magnitude of impact, followed by Halodule uninervis. Seagrasses in the low light treatment were impacted more slowly and to a lesser degree than the very low light treatment, therefore, efforts to minimize the degree of light reduction (e.g. better water quality) will be rewarded with reduced and delayed impacts to seagrasses.

  2. Temperature in shallow water seagrass habitats was measured over a one year period and water temperature reached a maximum of 43°C during low tide. In aquaria, the impact of short-term (2.5 hrs) spikes in water temperature to 35°C, 40°C and 43°C was measured for six days and one day recovery.

    • At 35°C there was a small, mostly positive, effect on the photosynthetic efficiency (FPSII, a), with the largest response occurring on high-light days. However, the time spent at saturating irradiance (Hsat) was either not affected or it was reduced. There were no detectable ecologically-relevant effects on growth or shoot density.
    • At 40°C, the temperature effect on the photosynthetic apparatus was slightly negative (reduced FPSII, and Hsat), but this temperature represents a critical and ecologically-relevant threshold as there were strong differences between species in their responses and there was a large impact on growth and shoot density.
    • At 43°C all species were strongly and negatively affected, with no survival after 2 to 3 days.

    Halophila ovaliswas the most sensitive to the spikes in temperature, and Cymodocea rotundata was the most resilient. Stressful temperatures (40-43°C) are already reached under current conditions, albeit not usually on consecutive days, and future increases in the frequency and intensity of such events will have significant ecological consequences for shallow seagrass meadows.

  3. The effects of both water temperature and light levels were tested on two species of seagrass that have different distributional ranges, but which overlap only in the Great Barrier Reef, Australia: Halodule uninervis is a tropical species common throughout the Indo-Pacific and east-African coast, while Zostera muelleri occurs in subtropical and temperate waters of Australia and New Zealand. The effect of three temperatures 27°C, 30°C and 33°C and two light levels – high light (400 µmol photons m-2 s-1) and low light (40 µmol photons m-2 s-1) – were tested in aquaria experiments. For the fully tropical species H. uninervis, temperature elevations to 33°C led to increased photosynthetic rates, while respiration rates remained constant. At low light levels, all temperature treatments had neutral shoot-scale production (photosynthesis less respiration), and reduced growth and biomass compared to high light. For Z. muelleri, a temperature change of 30°C to 33°C led to a critical metabolic imbalance – with large reductions in net shoot-scale production. Low light also had a significant impact on this species with reduced shoot-scale production, and reduced growth and biomass. The northern Australian population of H. uninervis appears to exist well within its optimal temperature range and, following future rises in sea temperature, could continue to thrive in the Great Barrier Reef region. In contrast, Z. muelleri currently exists near its upper thermal threshold. A temperature rise of 3°C by 2100 would likely to see the contraction of this species from the tropical regions of the Australian coastline. This could have ecologically significant ramifications, as Z. muelleri is often the only species able to inhabit muddy habitats.