Elsevier

Ecological Indicators

Volume 81, October 2017, Pages 578-586
Ecological Indicators

Capacity of a temperate intertidal seagrass species to tolerate changing environmental conditions: Significance of light and tidal exposure

https://doi.org/10.1016/j.ecolind.2017.04.056Get rights and content

Highlights

  • Z. muelleri can tolerate light stress during air exposure through photo-acclimation.

  • Synergistic relationship between light and tidal exposure for intertidal seagrasses.

  • Seagrasses with varying light history respond differently to increased irradiances.

  • Site-specific studies essential to inform management decisions.

Abstract

Seagrass meadows perform several important ecological roles in coastal areas. However, with multiple stressors threatening these aquatic plants, their current rate of decline is likely to increase, so understanding their ability to acclimate to a range of environments may be key to their survival. Light availability is critical for seagrass health, with loss of water clarity likely to cause decline. This study assessed the ability of the temperate intertidal species, Zostera muelleri, to compensate for variations in light in its natural environment. The impacts of light during inundation (high tide) and exposure (low tide) were examined along a vertical gradient from the high to low intertidal at two sites with differing light histories. Photo-acclimation was evaluated through morphological and physiological characteristics over two tidal cycles. Results were consistent with seagrasses having optimized their photosynthetic capacity, with physiological acclimations being site specific. Longer-term morphological changes were also noted, suggesting a dissimilar light history between sites for an extensive period. Interestingly, at the site with the greatest light range, but the lowest light penetration at depth, a significant reduction in photosynthetic activity during air exposure was observed. This suggests that Z. muelleri is capable of tolerating light stress through photo-acclimation, allowing for the more efficient harvesting of light at low levels. Overall, this study demonstrates that Z. muelleri has the ability to adjust both physiologically and morphologically to changing environmental conditions, a key aspect to survival and persistence in temperate intertidal zones.

Introduction

Marine embayments are often comprised of soft sediment habitats dominated by seagrass meadows. These high-value ecosystems are worth an estimated US$1.9 trillion to global markets each year in terms of nutrient cycling, with their support for commercial fisheries worth as much as US$3500/ha per year (Waycott et al., 2009). Seagrass habitats play a vital role in supporting ecosystems as they oxygenate water, regulate nutrients, stabilize sediments, provide nursery grounds for recreationally important fisheries, and are an essential food source for dugongs and turtles (Collier et al., 2012, Connolly, 2009). Unfortunately, multiple stressors (e.g. environmental, biological and climatological) are threatening these aquatic plants in many temperate and tropical ecosystems (Collier and Waycott, 2009). Seagrass meadows have been declining since 1990 at a rate of 7% per annum (Waycott et al., 2009), with up to 35% of worldwide seagrass beds significantly impacted (Nagelkerken, 2009).

Fluctuations in light, temperature, nutrients and substrate suitability severely impact primary production and cause seagrass decline (Connolly, 2009), particularly reductions in light availability (Collier et al., 2011). In addition to minimum light requirements for most seagrass species being within the range of 2–37% of surface irradiance (Lee et al., 2007), seagrasses that grow within the intertidal zone are often subjected to oversaturating irradiances which can also cause seagrass decline through thermal stress, desiccation and photo-inhibition (Petrou et al., 2013). As such, seagrass survival depends on their ability to acclimate to site-specific conditions (Silva and Santos, 2003), with an understanding of light thresholds crucial for effective management (York et al., 2013).

To ensure a constant adjustment to current light conditions, the physiological responses of seagrasses (such as maximal quantum yield (Fv/Fm), maximum electron transport rate (ETRmax) and photosynthetic light harvesting efficiency (α) can be altered within minutes or even seconds (Ralph and Gademann, 2005). Over the past decade, studies have provided evidence of adjustment through physiological (e.g. Campbell et al., 2003, Collier et al., 2009, Larkum et al., 2006, Silva and Santos, 2003) and morphological (e.g. Collier et al., 2007, Collier et al., 2009, Longstaff and Dennison, 1999, Olive et al., 2013) characteristics. Photosynthetic processes of seagrasses are highly responsive, with chlorophyll fluorescence techniques (e.g. Pulse Amplitude Modulated (PAM) fluorometery) allowing for measurements of real-time changes (Campbell et al., 2007). As chlorophyll fluorescence is inversely correlated with photosynthetic efficiency, such measurements are a useful tool for detecting physiological stress (Krause and Weis, 1991). Additionally, the overall photosynthetic performance of a leaf and its capacity to tolerate short-term changes in light can be determined using rapid light curves (RLCs) with associated derived parameters used (α and rETRmax) (Ralph and Gademann, 2005). These measurable responses are often the first evidence of acclimation (Maxwell et al., 2014), a form of plasticity that involves the modification of morphological and/or physiological characteristics in relation to changes in the environment (Angilletta, 2009).

To date, the majority of seagrass research has focused on the effects of low light on growth and photosynthesis, with few studies examining the impact of high light levels (Schubert et al., 2015). Of those investigations conducted, responses to higher irradiances demonstrate a reduction in photosynthetic capacity and significant photo-inhibition (e.g. Petrou et al., 2013, Ralph and Burchett, 1995). However, with light regimes fluctuating as a function of time of day, tidal schedule and vertical distribution, it is difficult to confidently predict the response of a population within a site. Therefore, this study compared the acclimation abilities of an intertidal seagrass species growing within two ecological niches (light regimes), located within the same temperate embayment.

The objective of this study was to investigate the capacity of an intertidal species to tolerate changing environmental conditions, by quantifying the influence of light and tidal exposure on physiological and morphological characteristics. Specifically, we measured photosynthetic characteristics, elemental ratios, isotope composition, genetic diversity and changes in seagrass cover and morphology. We focused on the seagrass species, Zostera muelleri, the dominant intertidal species in temperate Australia (Green and Short, 2003). With several climate models suggesting future increases in sea level rise and air temperatures (CSIRO, 2015), understanding how this species tolerates changing environmental conditions, such as light, is critical to predicting the effects of climate change and developing effective management strategies.

Section snippets

Methods

This study was conducted during summer at two sites on opposite sides of Western Port, a large embayment in southeastern Australia (Crib Point (38°22′26.6″ S, 145°13′22.9″ E) and Coronet Bay (38°27′57.0″ S, 145°25′29.6″ E)). Within the bay Z. muelleri plants are present from the high tide mark to the low tide mark. Western Port was chosen as a study site due to its previous history of extensive seagrass loss. In the early 1980’s losses of up to 75% were reported (Melbourne Water, 2011), with

Photosynthetically active radiation (PAR)

Average benthic daily light at both sites (within the high intertidal zone) differed during the measurement period depending on month (May 2014–May 2015). At Crib Point average benthic daily light over the 12-month period ranged from 6 mol m−2 d−1 to 24.8 mol m−2 d−1 and at Coronet Bay between 3 mol m−2 d−1 to 38.7 mol m−2 d−1. At both sites light levels were higher during January 2015 with levels significantly higher at Coronet Bay (M: 38.7 mol m−2 d−1, SD: 13.84 mol m−2 d−1) compared to Crib Point (M:

Discussion

This study demonstrates that Zostera muelleri has the ability to acclimate to changing environmental conditions and highlights the synergistic relationship between light and tidal exposure. By exploring the acclimation abilities of Z. muelleri growing within two light environments, this study highlights the species’ plasticity to a range of site conditions. Responses of seagrasses to reduced light regimes is well documented with the importance of minimum light requirements for growth and

Funding

This study was funded through the Australian Research Council Linkage Project (LP130100684) between Monash University, Melbourne University, Melbourne Water, Parks Victoria and the Environmental Protection Agency.

Acknowledgments

Special thanks to R. Coleman (Melbourne Water) and B. Sullivan for comments on a draft of the manuscript. Thanks to S. Koh and S. Day for assistance in the field. Thanks to P. Manassa for graphical input. Thanks also to the Victorian Marine Science Consortium for providing logistical support and facilities.

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