Abstract collection session 4: Coastal Development and Hinterland Dynamics (PA3)


Tropical beaches as biogeochemical groundwater filters

by Till Oehler and Nils Moosdorf

Submarine Groundwater Discharge | Submarine Groundwater Discharge

Beaches occur widespread in tropical regions and their importance, e.g. for tourism, is well known. Their role as a buffer zone along the land ocean continuum against inputs of pollutants from the hinterland into the coastal ocean has in turn been neglected so far. Tropical regions are known to be hotspots for groundwater discharge and associated inputs of biogeochemical active compounds such as nutrients into the coastal ocean, which can affect sensitive coastal ecosystems such as seagrass beds or coral reefs, and lead to eutrophication or the occurrence of harmful algae blooms. Before discharge into the ocean, groundwater often flows through the so called subterranean estuary (STE), a mixing zone between groundwater and seawater. In the recent years, this mixing zone has received increasing scientific attention as a biogeochemical reactor, which might be an important but still unexplored filter of pollutants. We therefore carried out a detailed investigation of a STE at Varkala Beach (India). The area belongs to India’s most touristy regions including a high anthropogenic stress on the coastal zone. Electrical resistivity tomography data coupled to detailed sampling of beach groundwater revealed that fresh groundwater flows from the hinterland through the beach into the ocean. Sediment incubation experiments, nutrient concentrations and stable isotopes of nitrate revealed that beach groundwater receives nitrogen from anthropogenic sources such as manure and/or sewage in the hinterland. The STE effectively removed nitrate via denitrification and released PO4 from the mineralisation of organic matter into groundwater. As a consequence, Varkala Beach lowers N/P ratios of discharging groundwater and thereby positively affects the biogeochemistry of its coastal ocean. We herein show that tropical beaches provide important ecosystem services which have so far been almost entirely neglected.


Coastal development impairs functioning, services and connectivity of downstream ecosystems – the example of Hainan, China

by Tim Jennerjahn

ZMT

The tropical island of Hainan is the largest special economic zone of China and its coasts are lined with mangrove forests, seagrass meadows and coral reefs. The beauty of these coastal ecosystems founds their economic potential for tourism, which is a major and growing industry sector. However, activities in other major economic sectors, i.e. agriculture, aquaculture and urbanization/industrialization, affect and impair the integrity of the coastal ecosystems.

Since the 1960s mangroves were massively deforested and to a large extent converted into aquaculture ponds on Hainan. Brackish water aquaculture, with intensive use of artificial fertilizers and feed as well as feeding with trash fish originating from artisanal capture fisheries, was and is a major pillar of Hainan’s economy. At the turn of the millennia the pond area reached its present-day areal extent. Untreated wastewater including large amounts of anthropogenic nitrogen is either released into semi-enclosed coastal bays or directly into coastal back-reef areas where it leads to eutrophication. Seagrasses are strongly impaired, abundance and diversity are declining, in some places they disappeared totally. Our data sets allowed to define a threshold of long-term exposure to dissolved inorganic nitrogen of 8 µM for seagrass survival. Coral reefs are similarly affected by eutrophication and other threats. We traced the pathway of anthropogenic nitrogen from aquaculture ponds into coastal waters and sediments as well as into the food web over four trophic levels by stable nitrogen isotope analysis. The anthropogenic nitrogen enrichment in the coastal zone is also documented in an age-dated sediment core.

Our long-term research in the interdisciplinary collaborative Sino-German projects LANCET, ECOLOC and TICAS enabled us to delineate the causes and consequences of coastal development for ecosystem functioning, services and connectivity in Hainan’s coastal zone. We established a dialogue with stakeholders and we propose measures to preserve the valuable coastal ecosystems.


Megaprojects in the Coastal Zone of Bangladesh: Impacts and Conflicts

by Marion Glaser and Samiya Selim

ZMT | ZMT, ULAB

Some of the clearly undesirable outcomes of human activities in water catchment areas are eutrophication, pollution, and reduced and less diverse marine ecosystems. Riverine and subsurface water flows transfer such impacts of human interventions on coastal lands into marine regions and thus further impacting ecosystem services and the well-being of coastal people. The coastal tropics are hotspots of such adverse social-ecological change and of related conflicts.

The drivers of such dynamics are often described as rooted in the needs and life styles of growing human populations. However, across the globe, economic growth and more recently Blue Economy and Blue Growth policies are also affecting coastal regions.  The “Blue Economy” agenda frames the ocean as the new economic frontier, and is attracting large amounts of new multi-national capital for large high investment and high impact “megaprojects”. This presentation reports on early work in the No CRISES project in Bangladesh. We focus on megaprojects and their role in changing coastal human-nature relations and report first fieldwork results with two groups of key stakeholders of a set of large projects in coastal Bangladesh: representatives of powerful economic actors (e.g. large companies, parastatals) and small-scale coastal fishers. Focusing on major conflicts as perceived by diverse stakeholders, we investigate the role and the potentials of largecoastal  projects for social-ecological change in the context of ongoing transformation. We report first results for field research in three regions of coastal Bangladesh which was carried out in November 2020.


Baiting Sharks into their ‘Right Place’: Marine Spatial Planning in a More-Than-Human Ocean

by Kon Kam King Juliette and Riera Léa

ZMT & IRD | IRD

Marine Spatial Planning (MSP) has been brought forward in the last decade as a promising management tool to sustainably organize and balance in space and time oft-competing claims over marine spaces and resources. Drawing upon a deepening understanding of anthropogenic pressures on the sea, MSP is often mobilized for the management of humans and their marine and coastal activities (including fishing and conservation ones) with relatively little recognition for non-humans’ agency. The aim of this communication is to propose a less anthropocentric angle and to interrogate the contours and implementation of MSP in “more-than-human” oceanic worlds.

Through the qualitative analysis of two case studies, one focusing on shark-oriented ecotourism in Fiji and one pertaining to shark risk management in New Caledonia, we discuss how the ‘right place’ of sharks and humans at sea is negotiated, defined and enforced by humans and non-humans in coastal areas. As we compare the practices deployed to control sharks and humans’ behaviors and whereabouts, we argue that sharks are increasingly incorporated into MSP (which they frame in return), although with distinct patterns of sharks-humans relations.

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