Photosynthesis and carbon balance in deep Posidonia oceanica meadows under the influence of diffuse anthropogenic pressures
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1
Oceanographic Center of Murcia, Spanish Institute of Oceanography, Spain
Cultural eutrophication has been identified as one of the main causes of seagrass decline worldwide. The discharge of nutrients and particulate materials into coastal waters by human activities has the potential to trigger direct and indirect interactions in seagrass communities, with the potential to impact seagrass fitness and survival, mainly through light limitation (Burkholder et al., 2007). Slow-growing species, such as the Mediterranean endemic species Posidonia oceanica, can be particularly sensitive to light reductions due to its high light requirements (Ruiz and Romero, 2001). This sensitivity is expected to be critical in deep P. oceanica meadows, where light levels are close to the minimum light requirements of the species (Ralph et al., 2007, Beer et al. 2014).
In order to protect these valuable habitats from eutrophication, European and National environmental agencies are moving those activities responsible of nutrient enrichment impacts (e.g. urban sewages and aquaculture) away from the shoreline and seagrass habitats. However, there is no empirical evidence about “safety distances” and, the relocation of such activities might not be sufficient to warrant the absence of impacts on seagrass meadows. The high potential of these pressures to exert a remote influence on nearby deep seagrass meadow limits have, indeed, already been evidenced (Ruiz et al. 2010). However, most studies evaluating the effects of nutrient enrichment on these habitats have been conducted at depths where light availability is not as limiting as in deep meadow margins. Consequently, there is a lack of scientific knowledge on how offshore discharges can impact the integrity and survival of deep P. oceanica meadows. The ability of deep meadows to cope with such impacts will largely depend on the physiological and metabolic plasticity of the species to minimize metabolic carbon unbalances.
The aim of this study was to characterize photoacclimative and metabolic responses of deep P. oceanica plants under the remote influence of offshore anthropogenic discharges. Five stations were distributed along the deep limit (26-28 m) of an extensive meadow (from north to south, A, B, C, D and E stations) following a gradient of anthropogenic pressure caused by the discharge of organic wastes (i.e. urban sewage and aquaculture facilities; Figure 1). Photosynthesis-Irradiance curves and daily light curves were characterized at each site to estimate the metabolic carbon balance of plants through the integration of photosynthetic variables and light regimes. The metabolic status of plants was also evaluated on the basis of carbohydrate concentrations in plant tissues. Determinations were conducted in summer and autumn, two different seasons of the annual growth cycle of the species, during which photosynthesis and carbon storage are at their maximum.
Figure 1. Map of the study area showing the location of sampling stations (A-E) and the main sources of nutrient enrichment: the aquacualture facility (about 7.000 tons year-1) and the pipeline through which urban wastes are delivered to offshore waters.
Results showed that light levels in sites close to the pressure sources were significantly lower (20% in station A and 34% in station B) than in sites away the influence of the discharges (control stations D and E). In summer, light availability in all stations was above the minimum light requirement of this seagrass species (10-16% relative to the surficial irradiance Io; Ruiz and Romero, 2001) but during autumn light levels in stations A and B were below such threshold (9.07-6.82% Io). This limited availability of light caused some modest but negative effects on the photosynthetic performance of plants (e.g. reduced net photosynthetic rates, effective quantum yield and relative ETR). Station B was the only site where some photo-acclimatory effort was observed in autumn plants, as reflected the significant increment in compensation irradiance (Ic). Carbon balance of summer plants was highly positive in all stations but contrarily, it was significantly reduced in autumn plants, whose respiration rates almost doubled those of summer plants. In particular, the carbon balance of autumn plants from stations A and B was significantly lower than in control stations (from 54 to 173%). It was negative in station A but slightly positive in station B, despite the fact that light reduction was more pronounced in this latter station. This result could be attributed to the photoacclimative mechanisms activated by plants from station B during autumn. As a result of the light reduction caused by organic discharges, the energy status (i.e. carbohydrates accumulation in rhizomes) was lower in plants from stations A and B during summer and only in station B during the autumn season.
Figure 2.Mean (standard error) of compensation irradiance (Ic) and metabolic carbon balance (CB) in sampling stations A-E in summer (grey bars) and autumn (black bars). Different letters indicate groups of homogeneus means obtained in the post-hoc test SNK (p<0.05).
In summary, our results suggested the influence of remote pressures on stations A and B. Alteration of light regimes in these sites is consistent with some photosynthetic responses and with the weakening of carbohydrate reserves of affected plants, although such responses also evidenced the low plasticity of the species to cope with reduced light conditions. The alteration of the carbon metabolism had no impact on the carbon balance of summer plants, but it affected autumn plants mainly due to their high respiratory rates. All these effects pointed to a reduced resilience of deep seagrass meadows to overcome subsequent light reductions during the winter, which is consistent with the general plant decline observed in the last years in stations A and B. In conclusion, this work evidenced the high vulnerability of deep P. oceanica meadows to remote, diffuse pressures and hence the urgent need to determine and to implement safety distances between organic wastes discharges and these valuable and vulnerable habitats.
Acknowledgements
This research has been funded by the POSINET project of the Spanish Institute of Oceanography and the Department of Fishery and Aquaculture of the Regional Ministry of Agriculture and Water of the Murcia Region: Monitoring network of Posidonia oceanica meadow and climate change of the Murcia Region.
References
Beer, S., Bjork, M. andBeardall, J. (2014). Photosynthesis in the Marine Environment. John Willey and Sons, 208 pp.
Burkholder, J. M., Tomasko, D. A. andTouchette, B. W. (2007). Seagrasses and eutrophication. Journal of Experimental Marine Biology and Ecology, 350(1), 46-72.
Ralph, P. J., Durako, M. J., Enriquez, S., Collier, C. J. and Doblin, M. A. (2007). Impact of light limitation on seagrasses. Journal of Experimental Marine Biology and Ecology, 350(1), 176-193.
Ruiz, J.M., Marcos, C., Sánchez-Lizaso, J.L.. (2010). Remote influence of off-shore fish farm waste on Mediterranean seagrass (Posidonia oceanica) meadow. Marine Environmental Research, 69: 118-126.
Keywords:
seagrass,
Mediterraenan sea,
Light reduction,
Urban sewage disposal,
Aquaculture,
photoaclimation,
carbon balance
Conference:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.
Presentation Type:
Poster Presentation
Topic:
Ecology, Biodiversity and Vulnerable Ecosystems
Citation:
Gimenez-Casero
J,
Conde-Caño
M,
Bernardeau-Esteller
J,
Garcia-Muñoz
R,
Ramos-Segura
A,
Marin-Guirao
L and
Ruiz
J
(2019). Photosynthesis and carbon balance in deep Posidonia oceanica meadows under the influence of diffuse anthropogenic pressures.
Front. Mar. Sci.
Conference Abstract:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) .
doi: 10.3389/conf.fmars.2019.08.00178
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Received:
25 May 2019;
Published Online:
27 Sep 2019.
*
Correspondence:
Dr. Jaime Bernardeau-Esteller, Oceanographic Center of Murcia, Spanish Institute of Oceanography, Murcia, Spain, jbernardeau@gmail.com