Distinguishing downregulation from other non-photochemical quenching of an Antarctic benthic macroalga using<i>in situ</i>fluorometry

Some ecosystems can undergo abrupt transformation in response to relatively small environmental change. Identifying imminent 'tipping points' is crucial for biodiversity conservation, particularly in the face of climate change. Here, we describe a tipping point mechanism likely to induce widespread regime shifts in polar ecosystems. Seasonal snow and ice-cover periodically block sunlight reaching polar ecosystems, but the effect of this on annual light depends critically on the timing of cover within the annual solar cycle. At high latitudes, sunlight is strongly seasonal, and ice-free days around the summer solstice receive orders of magnitude more light than those in winter. Early melt that brings the date of ice-loss closer to midsummer will cause an exponential increase in the amount of sunlight reaching some ecosystems per year. This is likely to drive ecological tipping points in which primary producers (plants and algae) flourish and out-compete dark-adapted communities. We demonstrate this principle on Antarctic shallow seabed ecosystems, which our data suggest are sensitive to small changes in the timing of sea-ice loss. Algae respond to light thresholds that are easily exceeded by a slight reduction in sea-ice duration. Earlier sea-ice loss is likely to cause extensive regime shifts in which endemic shallow-water invertebrate communities are replaced by algae, reducing coastal biodiversity and fundamentally changing ecosystem functioning. Modeling shows that recent changes in ice and snow cover have already transformed annual light budgets in large areas of the Arctic and Antarctic, and both aquatic and terrestrial ecosystems are likely to experience further significant change in light. The interaction between ice-loss and solar irradiance renders polar ecosystems acutely vulnerable to abrupt ecosystem change, as light-driven tipping points are readily breached by relatively slight shifts in the timing of snow and ice-loss.

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“…Two to eight individual thalli of a species were sampled at each visit. See Runcie & Riddle () for further methodological details.…”

Section: Methodsmentioning

confidence: 99%

“…The three species were then examined in situ at depths of 6–9 m using autonomous logging fluorometers (Runcie & Riddle, ). Φ II ′ and irradiance were regularly measured every 90 min over 24‐h intervals, and oxygen evolution rates were calculated for each time point.…”

Section: Methodsmentioning

confidence: 99%

Light‐driven tipping points in polar ecosystems

Clark

Stark

Johnston

et al. 2013

Global Change Biology

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118

137

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“…While our multichannel fluorometer (incorporating a Junior-PAM fluorometer, Walz, Germany; see Runcie & Riddle, 2004) brought multi-sample fluorescence monitoring to underwater systems, the more recent development and implementation of specialized autonomous submersible fluorometers (e.g. Shutter Fluorometer, Aquation, Australia) has enabled an increase in both the temporal resolution and replication of in situ macroalgal and seagrass fluorescence (Runcie et al, 2009;Runcie & Riddle, 2011).…”

Section: Introductionmentioning

confidence: 99%

“…Thalli were positioned in plastic leaf clips that ensured a fixed distance between sample and fluorometer (see fig. 1 in Runcie & Riddle, 2011). Generally, measurements of the effective quantum yield of PSII fluorescence ( PSII 0 ) were made halfway along the length of the thallus to avoid juvenile and distal necrotic tissue.…”

mentioning

confidence: 99%

“…and H. grandifolius were conducted in situ using either the multichannel fluorometer, or with 'sidefire' or 'shutter' custom fluorometers (Runcie et al, 2009;text and fig. 1 in Runcie & Riddle, 2011). The custom fluorometers were designed independently of the Walz instruments, and later versions (Shutter Fluorometer and Submersible Datalogger) are now manufactured by Aquation, Umina Beach, Australia.…”

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confidence: 99%

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Estimating primary productivity of marine macroalgae in East Antarctica usingin situfluorometry

Runcie

Riddle

2012

European Journal of Phycology

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Photosynthetic activity of marine macroalgae in the Windmill Islands, East Antarctica, was measured in situ using submersible modulated fluorometers. An empirical relation incorporating terms for respiration and non-linear electron transport was derived from simultaneous in vivo measurements of effective quantum yield ( PSII 0 ) and oxygen evolution. This relation was used with in situ measurements of PSII 0 and photosynthetic photon flux density acquired over 24-h periods to estimate oxygen evolution rates of algae over the course of the measurement period. Productivity ranged from À8 to 19 mmol O 2 g À1 FW h À1(FW ¼ fresh weight), with daily carbon gain ranging from À1.5 to 3.6 mg C g À1 FW d À1 for midnight ice-covered algae and midday ice-free algae, respectively. These values were similar to published values of productivity of Antarctic species derived from oxygen-and carbon-based techniques. The technique described here provides a simple and rapid means of estimating primary productivity in marine systems.

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