Capítulo 4. Resultados
4.2 Modelo 2. Casos 2 al 7
In the following, the main sources, pathways and sinks affecting nutrients and OM in the WWE and nearshore coastal back-reef areas under low to moderate precipitation conditions are discussed and summarized in Tab. 3a. Additionally, the main biogeochemical processes involved are illustrated in a simplified sketch (Fig. 6).
Fig. 6: Simplified sketch summarizing the major biogeochemical processes in the WWE under low-moderate precipitation as presented in CHAPTER V (Herbeck et al., 2011).
IN THE WENCHANG/WENJIAO ESTUARY (WWE)
The composition of nitrogen and į15N of NH4+, NO3- and TSM suggests two main sources affecting the nutrient and OM of the WWE under low to moderate rain conditions.
A relatively high NO3- contribution (~55%) of DIN and low į15N values of the TSM (~1.5‰) in the upper estuary reveal an input of nutrients from agriculture into the rivers Wenchang and Wenjiao (CHAPTER II, V), since effluents from agriculture fields fertilized by artificial fertilizers are usually high in NO3- (Mian et al., 2009) and have a low į15N of -2 to 2‰ (Lee et al., 2008). Additionally, benthic nutrient fluxes from the sediment as a result of strong benthic recycling contribute as internal source to the
observed high nutrient concentrations in the upper catchment area (CHAPTER V). This is conceivable because fractionation during remineralization of the high OM inputs into the sediment would likely also lead to fluxes of DIN with a low į15N.
In contrast, a predominance of NH4+ (~80%) in the DIN pool of large parts of the downstream estuarine area points to NH4+ and DON from shrimp and fish ponds as main sources, as these are the dominant nitrogen components in their effluents (CHAPTER I, II, V). Nitrogen in pond effluents was usually heavily enriched in 15N due to volatilization of ammonia (NH3) and other discrimination processes during recycling of OM from feed and faeces that accumulate in shrimp and fish ponds over crop production (CHAPTER VI). Values of ~17‰ į15N-NH4+ and ~7‰ į15N-NO3- are the first existing measurements from aquaculture ponds, and confirmed the assumptions of previous studies that pond effluents are enriched in 15N (CHAPTER VI). Preliminary data of the į15N-NH4+ and į15N-NO3- in waters of the WWE (Fig. 7), which were on similarly high levels than in aquaculture effluents, suggest aquaculture effluents to be the dominant nitrogen source in the estuarine lagoon.
Fig. 7: Distribution of į15N-NH4+ and į15N-NO3- in the Wenchang/Wenjiao Estuary, NE Hainan, during July/August 2008 and March/April 2009. Samples were taken under low to moderate rain conditions.
Theoretically, it is also conceivable that untreated municipal effluents as well as runoff from agriculture fields that were fertilized with manure added to the high nutrient and OM loads from aquaculture effluents, because both potential sources are also known to mainly consist of NH4+ and to be enriched in 15N (Jones et al., 2001).
However, if municipal effluents were of importance in the area, they would rather have affected the upper estuary near the cities Wenchang and Wenjiao, where nutrient composition and į15N reflected agriculture as the main source. Effluents from fields fertilized with manure are unlikely to be the source of the high nutrients and OM, as artificial fertilizers based on N-fixation are rather applied on the fields in the catchment area than manure (pers. comm. local farmers). With simultaneous consideration of the large area of the estuarine lagoon that is covered by shrimp and fish ponds (~20 km²), from which an estimated volume of 210 *106 m³ effluents is annually released (CHAPTER I), it is concluded that aquaculture effluents were the predominant source accounting for the nutrient concentrations. It is conceivable, though, that wastes from fish net cages, which cover ~5 ha in the estuarine lagoon contribute to the pond-based aquaculture effluents.
Uptake of the nutrients by phytoplankton was the main transformation process in the upper estuary and the estuarine lagoon. In-situ production thus represents a major sink for the nutrients, while at the same time it is the main source of the OM in the WWE. This was indicated by the primarily autochthonous origin of the TSM, as indicated by a C/N ratio close to the Redfield ratio of 6.7 (Redfield et al., 1963), į13Corg of the TSM ranging from -22 to -32‰, typical for a mixture of marine (-18 to -22‰;
Fischer, 1991) and fresh water phytoplankton (-29 to -32‰; Martinelli et al., 1999), as well as the composition of particulate amino acids and hexosamines (IV, V). In contrast, the contribution of soils and sediments was generally low, only slightly increasing towards the outlet of the estuarine lagoon due to tidal-driven resuspension of sediments (CHAPTER IV). Phytoplankton biomass is sustained by agriculture derived nutrients, especially NO3-, in the upper estuary as indicated by the low į15N (~1.5‰), while uptake of aquaculture-derived nutrients, especially 15N-enriched NH4+
as well as inputs of TSM high in į15N with effluents from aquaculture ponds resulted in the high į15N values of TSM of usually >7‰ in the estuarine lagoon (CHAPTER V).
The distinct spatial difference between sources of nutrients and OM between the upper catchment area and the estuarine lagoon was most likely derived from immediate consumption of large parts of the in-situ production (Maier, 2010) from agriculture derived sources in the upper estuary (CHAPTER V). Therefore, the impact of effluents from agriculture fertilizers is mainly restricted to the upper catchment area, while impacts of nutrients from aquaculture ponds predominate in the estuarine lagoon of the WWE.
Besides phytoplankton, other primary producers such as remaining mangroves and water hyacinths also take up the nutrients from aquaculture effluents, as reflected by the elevated į15N values in the plant tissues (Tab. 2a; CHAPTER V). Uptake by
primary producers resulted in decreasing nutrient concentrations along the estuarine downstream gradient. Additional processes reducing nutrient concentrations are tidal driven dilution with nutrient-poor marine waters, as indicated by high salinities in the estuarine lagoon (CHAPTER I, II, III, IV, V), as well as PO43- removal due to its adsorption behaviour onto suspended particles (CHAPTER II). However, much lower nutrient concentrations than expected from linear mixing of marine and fresh water end-members points to uptake by phytoplankton and other primary producers as the primary process for nutrient removal along the estuarine gradient (CHAPTER V). In the WWE, nutrient uptake by primary producers is likely facilitated by an enhanced residence time due to the enclosed shape of the estuarine lagoon. The efficient consumption of nutrients within the estuarine lagoon leads to a restricted nutrient export from the estuary into coastal waters, despite the high nutrient inputs from aquaculture.
While parts of the phytoplankton biomass are likely exported into coastal waters with tidal movement, most of the phytoplankton biomass remains within the estuarine lagoon, where it is consumed, recycled or buried in the sediment (CHAPTER IV, V).
Decomposition of planktonic and aquaculture-derived OM likely displays an additional internal source of nutrients to estuarine waters. Burial of phytoplanktonic OM in estuarine sediments was evident from high į15N in sediments of the past 30 years (6-7‰) compared to older sediments originating from before 1990 with a į15N of 5-6‰
(Fig. 8). The high į15N values of recent sediments thus reflect burial of aquaculture derived nitrogen in estuarine sediments. Consumption of estuarine phytoplankton by primary consumers and further transfer up the food chain was revealed by high į15N of 10-14‰ in the estuarine consumers of the study area (Tab. 2a). į15N values usually increase by 3-4‰ with each trophic level (Minawage and Wada, 1984). For each trophic level, į15N values were higher than those reported from little affected estuaries (McClelland et al., 1997) and comparable to those reported from other estuaries affected by sewage effluents (e.g. McClelland et al., 1997; Schlacher et al., 2005;
Hadwen and Arthington, 2007), indicating that the aquaculture-derived nitrogen is transferred through all trophic levels of the food web. Considerable parts of the fish catches in the estuarine lagoon are not used for human consumption, but instead used as so called “trash fish” for feeding shrimps and fish in ponds (Krumme et al., subm.).
Their biomass is in parts recycled in the ponds and exported in form of nutrients and OM into estuarine waters. Thus, higher trophic level organisms, such as estuarine fishes may be sinks and sources of nutrients and OM passing the estuarine recycling loop again and again.
Tab. 2: į15N ranges in various organisms in the WWE (a) and back-reef areas (b) in NE Hainan
Organism Reference
a) In the estuarine lagoon of the WWE
phytoplankton 7 - 10 CHAPTER V
water hyacinths 7 - 12 CHAPTER V
mangroves 6 - 8 Herbeck, et al. (unpubl. data) crustaceans 7 - 8 Herbeck, et al. (unpubl. data) fishes (detrivores) 11 - 14 Krumme et al. (unpubl. data) fishes (herbivores) 11 - 14 Krumme et al. (unpubl. data) fishes (zooplanktivores) 11 - 15 Krumme et al. (unpubl. data)
b) In the coastal back-reef areas
seagrass 5 - 9 CHAPTER VI
epiphytes 7 - 10 CHAPTER VI
macroalgae 6 - 8 Scharfbillig (2009)
fishes 10 - 17 Scharfbillig (2009)
bivalves 8 - 9 Scharfbillig (2009)
gastropodes 7 - 9 Scharfbillig (2009) crustaceans 8 - 12 Scharfbillig (2009) polychaetes 11 - 14 Scharfbillig (2009)
į15N [‰]
Fig. 8: į15N along depth in a) a dated mangrove sediment core from the WWE (Bao et al., subm.), and b) a sediment core from the middle of Bamen Bay (Herbeck and Unger, unpubl. data)
IN THE COASTAL BACK-REEF AREAS
High į15N values of usually >8‰ of various primary producers in coastal waters close to the shore at the three back-reef areas of Ye Lin, Qingge and Chang qi gang, indicate the uptake of 15N enriched DIN (CHAPTER VI). Strong increases in į15N from
~6‰ to up to 13‰ over time of offshore phytoplankton incubated close to the shore in a bioassay experiment also proved that mainly the 15N enriched ammonium released from aquaculture ponds was taken up (CHAPTER VI). This implies that effluents from coastal aquaculture ponds that are directly released to coastal waters via drainage channels are a significant nutrient source for coastal back-reef areas and are responsible for the observed nutrient enrichment in coastal waters.
Decreasing į15N values in phytoplankton, seagrass and epiphytes in combination with water column DIN in offshore direction indicate a removal of pond derived nitrogen and increasing nitrogen contributions from unaffected marine sources.
Similar to estuarine waters, uptake by primary producers, including phytoplankton, benthic and epiphytic algae and seagrasses, as well as tidal-driven mixing with nutrient-poor ocean water were likely the main removal processes. Nevertheless, į15N in epiphyte and seagrass tissues higher than 5-7‰, the typical į15N value of seawater (Miyake and Wada, 1967; Wada et al., 1975), and significant elevation of į15N after incubation in a bioassay reflect the persisting impact of aquaculture-derived nitrogen at the stations furthest offshore. This, together with significant increases in į15N of incubated offshore water at these stations, indicates that aquaculture effluents affect the entire back-reef areas exceeding 1000 m from the shore in Qingge and 2500 m from the shore in Chang qi gang (CHAPTER VI). This is corroborated by findings of Roder et al. (subm.), who reported high į15N values in the tissue of the coral Porites lutea in five reefs in NE Hainan varying from 6.8-9.0‰ in coral host and 6.9-9.3‰ in zooxanthellae.
The average į15N measured in the seagrass T. hemprichii from the back-reef areas of Hainan is the highest ever measured in this species worldwide (Fig. 9).
Similarly, į15N values in epiphytes were the highest ever measured (CHAPTER VI).
This indicates that the seagrass meadows of NE Hainan are extremely exposed to aquaculture wastes.
Fig. 9: į15N in T. hemprichii leaf tissue
Comparable to inner-estuarine findings, the main OM sinks are sediment recycling and burial, as well as consumption by higher trophic levels. Indicative for that were high į15N values of sedimentary OM (7.5±0.4‰; n=12), as well as those of various consumers in the back-reef areas (Tab. 2b). Parts of the OM might have reached open ocean waters and were buried or processed there. Overall, similar to the estuarine lagoon, the back-reef areas of NE Hainan are substantially enriched with aquaculture derived nutrients.
3.1.3 Sources, pathways and fate of nutrients and organic matter under heavy