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Biogeochemistry of Mediterranean Wetlands: a Review About the Effects of Water-Level Fluctuations on Phosphorus Cycling and Greenhouse Gas Emissions

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Biogeochemistry of Mediterranean Wetlands: a Review About the Effects of Water-Level Fluctuations on Phosphorus Cycling and Greenhouse Gas Emissions water Review Biogeochemistry of Mediterranean Wetlands: A Review about the Effects of Water-Level Fluctuations on Phosphorus Cycling and Greenhouse Gas Emissions Inmaculada de Vicente 1,2 1 Departamento de Ecología, Universidad de Granada, 18071 Granada, Spain; [email protected]; Tel.: +34-95-824-9768 2 Instituto del Agua, Universidad de Granada, 18071 Granada, Spain Abstract: Although Mediterranean wetlands are characterized by extreme natural water level fluctu- ations in response to irregular precipitation patterns, global climate change is expected to amplify this pattern by shortening precipitation seasons and increasing the incidence of summer droughts in this area. As a consequence, a part of the lake sediment will be exposed to air-drying in dry years when the water table becomes low. This periodic sediment exposure to dry/wet cycles will likely affect biogeochemical processes. Unexpectedly, to date, few studies are focused on assessing the effects of water level fluctuations on the biogeochemistry of these ecosystems. In this review, we investigate the potential impacts of water level fluctuations on phosphorus dynamics and on greenhouse gases emissions in Mediterranean wetlands. Major drivers of global change, and specially water level fluctuations, will lead to the degradation of water quality in Mediterranean wetlands by increasing the availability of phosphorus concentration in the water column upon rewetting of dry sediment. CO2 fluxes are likely to be enhanced during desiccation, while inundation is likely to decrease cumulative CO emissions, as well as N O emissions, although increasing CH emissions. Citation: de Vicente, I. 2 2 4 Biogeochemistry of Mediterranean However, there exists a complete gap of knowledge about the net effect of water level fluctuations Wetlands: A Review about the Effects induced by global change on greenhouse gases emission. Accordingly, further research is needed to of Water-Level Fluctuations on assess whether the periodic exposure to dry–wet cycles, considering the extent and frequency of the Phosphorus Cycling and Greenhouse cycles, will amplify the role of these especial ecosystems as a source of these gases and thereby act Gas Emissions. Water 2021, 13, 1510. as a feedback mechanism for global warming. To conclude, it is pertinent to consider that a better https://doi.org/10.3390/w13111510 understanding about the effect of water level fluctuations on the biogeochemistry of Mediterranean wetlands will help to predict how other freshwater ecosystems will respond. Academic Editor: Pieter Boets Keywords: biogeochemistry; phosphorus; greenhouse gases emissions; sediment; water level fluctu- Received: 15 March 2021 ations; Mediterranean wetlands Accepted: 25 May 2021 Published: 27 May 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Wetlands are among the world’s most productive and valuable ecosystems [1]. Despite iations. covering only 1.5% of the Earth’s surface, wetlands provide 40% of ecosystem services worldwide, such as provisioning, regulating, habitat and cultural services [2]. Among them, three regulating services are globally significant [3,4], namely water quality improvement, flood abatement, and carbon management. Wetlands purify water (which is why they are often called ‘nature’s kidneys’) through storing nutrients and other pollutants in their soils Copyright: © 2021 by the author. and vegetation, and trapping sediments [5]. In particular, nutrients such as phosphorus and Licensee MDPI, Basel, Switzerland. nitrogen (as nitrate), commonly associated with agricultural runoff and sewage effluent, are This article is an open access article distributed under the terms and removed or significantly reduced by wetlands [6,7]. As a result, wetlands are traditionally conditions of the Creative Commons considered as “hotspot” for biogeochemical transformation worldwide [8]. Attribution (CC BY) license (https:// Biogeochemical processes in wetlands are highly dynamic [9,10] and involve complex creativecommons.org/licenses/by/ interactions between hydrological processes, mineralogical transformations, and bacterial 4.0/). and vegetation communities [11,12]. The increasing worldwide affectation of wetlands by Water 2021, 13, 1510. https://doi.org/10.3390/w13111510 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 20 transformation worldwide [8]. Biogeochemical processes in wetlands are highly dynamic [9,10] and involve com- Water 2021, 13, 1510 plex interactions between hydrological processes, mineralogical transformations,2 ofand 20 bacterial and vegetation communities [11,12]. The increasing worldwide affectation of wetlands by a shift in environmental conditions due to climate change, land use devel- aopment, shift in environmentalresource extraction, conditions urbanization, due to climate and se change,a level landrise use[13,14], development, will lastly resourcealter the extraction,hydrological urbanization, regime, impacting and sea levelthe biogeochem rise [13,14],ical will processes. lastly alter Although the hydrological some previous regime, impactingreviews have the biogeochemicalbeen conducted, processes.both under Although a global perspective some previous [15–17] reviews and haveunder been a re- conducted,gional context both [18], under the a present global perspective study is the [15 first–17 ]review and under focused a regional on Mediterranean context [18], wet- the presentlands, which study isare the among first review the most focused threatened on Mediterranean ecosystems wetlands,due to climate which change are among and thenat- mostural threatenedhabitat loss ecosystems [19]. Mediterranean due to climate wetlands change are and considered natural habitatas unique loss ecosystems [19]. Mediter- ex- raneanposed wetlandsto Mediterranean are considered climate as forcing, unique and ecosystems thus they exposed are located to Mediterranean in a more extensive climate forcing,region beyond and thus the they Mediterranean are located in basin, a more including extensive other region geographic beyond theareas Mediterranean such as parts basin,of California, including South other Africa, geographic Chile, areas and such Southern as parts Australia of California, [20] (Figure South Africa, 1). As Chile,a result and of Southernthis climate, Australia they are [20] subjected (Figure1). to As a a strong result ofseasonality this climate, in theywater are supply subjected outside to a strongthe hot seasonalityseason and in they water also supply experience outside a long the hot vegeta seasontion and period they that also makes experience them a quite long vegeta-distinct tionfrom period the standard that makes limnological them quite paradigm distinct from [21]. the Apart standard from limnological these external paradigm conditions, [21]. Apartseveral from internal these features external (i.e., conditions, shallowness several and internal a typically features high (i.e., catchment shallowness area to and lake a typicallyarea ratio) high make catchment Mediterranean area to lake wetlands area ratio) especially make sensitive Mediterranean to water wetlands level fluctuations especially sensitiveinduced toby water global level change. fluctuations induced by global change. FigureFigure 1. 1.Geographic Geographic locations locations with with a a Mediterranean Mediterranean climate. climate. InIn this this review, review, after after explaining explaining the vulnerabilitythe vulnerability of these of ecosystemsthese ecosystems to global to change global andchange the keyand rolethe playedkey role by played sediments by sediment in their biogeochemistry,s in their biogeochemistry, we analyze we the analyze potential the effectspotential of sucheffects hydrological of such hydrological changes in changes both phosphorus in both phosphorus dynamics dynamics and greenhouse and green- gas emissions.house gas Phosphorusemissions. Phosphorus is one of the is major one of nutrients the major limiting nutrients primary limiting production primary in produc- many freshwatertion in many ecosystems freshwater [22] ecosystems and, thus, it [22] is frequently and, thus, responsible it is frequently for eutrophication, responsible whichfor eu- istrophication, a critical issue which for water is a critical managers issue [23 for]. Our water additional managers interest [23]. Our is to additional assess the dependence interest is to ofassess greenhouse the dependence gas emissions of greenhouse under different gas emissions water level under scenarios. different This water concern level is scenarios. based on theThis observation concern is that based wetlands on the have observation the highest that carbon wetlands density have among the highest terrestrial carbon ecosystems density and contain around 15% of the world’s organic soil carbon [24], being the dominant natural among terrestrial ecosystems and contain around 15% of the world’s organic soil carbon source of methane emissions [25]. Accordingly, in the context of global change, it is [24], being the dominant natural source of methane emissions [25]. Accordingly, in the essential to clarify the last effect of hydrological fluctuations in Mediterranean wetlands on context of global change, it is essential to clarify the last effect of hydrological fluctuations the release or retention of sedimentary phosphorus and on their role as a sink or a source in Mediterranean wetlands on the release or retention of sedimentary phosphorus
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