1. Modelling intertidal seagrass meadows: physical influence on biological processes
Ana Azevedoª, 
Ana Lillebøb, João Lencart e Silvac, João Miguel Diasa 
a NMEC, Department of Physics, CESAM, University of Aveiro
b Department of Biology, CESAM, University of Aveiro
c Instituto Português do Mar e da Atmosfera, IPMA
Abstract
Seagrasses are marine plants which have key ecological roles in coastal ecosystems. In the present work, the complex interactions established between the surrounding physical environment and relative water content loss of seagrass leaves (RWC), during air-exposure periods at low tide, are studied through field surveyed data and formulation of a desiccation numerical model.
The experimental data showed no clear pattern between duration of air-exposure periods and RWC of seagrass leaves, though sediment grain-size seemed to have an important role to the variation of RWC. The following formulation of desiccation model took this into account, simulating the RWC according to tide, sediment type and different air temperature scenarios.
Further improvements to desiccation model performance may include additional field surveys for determination of critical parameters and, through coupling with biological and water quality models, provide a better comprehension on the importance of desiccation periods to the overall seagrass seasonal biomass cycle.
Seagrass biodiversity and biomass have been decreasing in Ria de Aveiro lagoon during the last decades (Silva et al., 2004). Seagrass numerical models are resourceful tools to better understand the complexity of these systems. In spite of the current efforts to model seagrass dynamics, the exposure periods of intertidal seagrasses to air during low tide remain highly overlooked. As so, Relative Water Content (RWC) dynamics of seagrass leaves were measured during a tidal cycle in a healthy intertidal seagrass meadow in Ria de Aveiro lagoon (Fig 1a and b) and a preliminary desiccation model was formulated to address its specificities, according to tide and sediment type.
The desiccation field experiment suggested that sediment grain size has a more expressive influence on the variations of RWC, rather than the exposure time itself (Fig 2a and b). In fact, no regular pattern between the RWC and different intertidal heights was identified (Azevedo et al., 2016). Seagrass leaves colonising medium-sized sand sediments showed higher RWC losses (20-31%), comparing to those colonising muddy/ fine sand sediments (8-16%).
The desiccation model simulated the Relative Water Content (RWC), depending on hydration and desiccation of seagrass leaves when exposed to air (Azevedo et al., 2017). The results showed that the module responded well to distinct tidal conditions (spring vs neap tides), different air temperature scenarios (maximum air-temperature registered in the field surveyed day – 20.6 ºC – vs sub-lethal temperature for Zostera noltei – 38 ºC) and sediment type based on grain size (Fig 3 a-d). Higher losses on RWC of seagrass leaves occur at spring tides, with higher air temperatures, longer air-exposure periods (i.e. during spring tides) and colonizing medium sized sand sediments, as suggested by the desiccation field experiments.
Ongoing improvements comprise the coupling of the aforementioned desiccation model with a Water Quality Model Delft3D-WAQ, previously applied to Ria de Aveiro lagoon (LAGOONS, 2012). As there is potential for further improvements, the wind speed effect on desiccation of intertidal seagrass leaves will be addressed, through the adaptation of terrestrial evapotranspiration methods as a proxy of seagrass leaves water content losses. Moreover, the planning of further desiccation field experiments will be critical to determine further parameters accounting for model improvements.
Conclusion
This work highlights the importance of applying numerical models when studying complex system interactions, as those comprising the biological and physical features of estuarine areas, including seagrass meadows and vicinity environment. The improvement of seagrass numerical models may provide comprehensive knowledge to support management actions to revert the current worldwide seagrass trends.
References
Azevedo A., Dias J.M., Lillebo A.I. (2016) Thriving of Zostera noltei under intertidal conditions: implications for the modelling of seagrass populations. Marine Biology. 163, 5
Azevedo A., Lillebø A.I., Silva J.L., Dias J.M. (2017) Intertidal seagrass models: insights towards the development and implementation of a desiccation module. Ecological Modelling. 354,
Duck, R.W., Catarino, J.B., Seagrasses and sediment response to changing physical forcing in a coastal lagoon. Hydrology and Earth System Sciences 8,
LAGOONS, Hydrodynamic and Water Quality Models. LAGOONS Report D pp.
Fig 1 Location of the study area a), with detail of respective seagrass meadow b). a b
Fig 2 Relative Water Content for seagrasses (RWCseagrass) during ebbing and flooding for a) Transect S – medium sand sediments, and b) Transect M – muddy fine sand sediments.
Fig 3 Modelled Relative Water Content (RWC) of seagrass leaves for different air temperature scenarios (20.6 ºC – a) and c) – and 38 ºC, b) and d)) and sediment types (muddy/fine sand sediments, a) and b), and medium sand sediments, c) and d)).
The first author has been supported by Fundação para a Ciência e Tecnologia (FCT) through a doctoral grant (SFRH/BD/84613/2012). Thanks are due for the financial support to CESAM (UID/AMB/ POCI FEDER ), to FCT/MEC through national funds, and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020.