## 1 Introduction  
Coastal ecosystems are under increasing stress from anthropogenic eutrophication. Enhanced land-ocean fluxes of dissolved nutrients can lead to phytoplankton community shifts and production increase, causing higher frequency of harmful algal blooms, and hypoxia and anoxia, with dramatic loss in benthic and pelagic biodiversity and a spread in so-called \"dead zones\" [1; 2; 3; 4; 5; 6; 7; 8]. These problems are particularly severe in the tropics and subtropics (e.g., [9; 10]) because many coastal ecosystems, like coral reefs and many seagrass areas, are particularly adapted to oligotrophic conditions and sensitive to nutrient pollution. In addition, human activity currently increases nutrient pollution more in developing countries in subtropical and tropical areas than in developed countries in higher latitudes, where nutrient management has evolved further. Despite this, low latitude regions are underrepresented in scientific research [11]. Mangroves covered nearly 167,000 km\({}^{2}\) in tropical and subtropical coastal regions at the beginning of the 21 st century [12]. Growing agriculture, aquaculture, and urban development led to widespread mangrove degradation. Global loss rates are about 1% per year and the total global mangrove area has been reduced by 35% since the 1980s (reviewed by [13]). More dramatically, in China, this number is close to 75% [12; 14]. This resulted in the loss of essential ecosystem services, including the filtration of anthropogenic pollutants. Mangroves mediate the land-ocean transport of nutrients [15] and may protect adjoining systems from eutrophication [16; 17; 18] caused by domestic sewage ([19; 20] and references therein), shrimp effluents [21; 22], and livestock and industrial waste [23; 24]. This pivotal importance in controlling coastal eutrophication and related consequences demands intensified mangrove conservation [16]. Benthic respiration and nutrient regeneration are essential processes in coastal and estuarine ecosystems, controlling organic carbon cycling and the flux of nutrient-type elements across the sediment-water interface [25; 26; 27]. In mangrove forests, rates of nutrient conversion, uptake, removal, and immobilization by benthic sedimentary processes can be higher than by than mangrove vegetation [28]. Pore water profiles of dissolved species can be used to estimate diffusive benthic fluxes relating to the concentration gradients that result from microbial activity in the sediment [29]. However, benthic macrofauna alter actual fluxes compared to diffusive transport through bioturbation, advection, and foraging activity [30; 31; 32; 33; 34]. Fluxes are further affected by uptake and release of nutrients by microphotobenthos and other microorganisms at the sediment surface [35; 36; 37]. Intact sediment core incubations yield reliable results of actual fluxes [38], and have been used in different environmental systems, including lakes (e.g., [39]), estuaries [40], coastal seas [41], and the deep sea [42]. Despite some criticism of the application of laboratory experiments [43], other studies found no differences between _in situ_ and _ex situ_ incubations [44]. Recent studies investigated total mangrove system sink and source function by comparing concentration differences between flood and ebb waters [16, 45, 46, 47, 48], rather than focusing on pore waters or interface exchange. In this study we combine these approaches to answer questions about (1) the total forest function; (2) the importance of sediment-water-fluxes; and (3) the underlying benthic processes in a south Chinese estuarine mangrove system. The Naniu River Estuary receives very high inputs of anthropogenic nutrients, bearing the potential for eutrophication of adjacent coastal waters [49, 50]. Aquaculture development has led to the loss of mangroves as natural filtration systems, with an area reduction from 1790 ha to 515 ha between the early 1990s and 2011 [14]. Furthermore, as is typical for coastal areas, the region is characterized by rapid population growth.