What do the most harmful substances in coastal waters of Hainan originate from?
Eutrophication
The effluents from aquaculture facilities are a major source of dissolved inorganic nutrients and organic matter leading to coastal eutrophication for more than 20 years. Brackish water aquaculture ponds are the major threat because of the large areal cover and long-term operation.
Eutrophication of natural water bodies caused by insufficiently treated effluents can result in pollution of
water resources used in the aquaculture industry.
For decades, eutrophication and associated risks such as oxygen and biodiversity decline have been recognized as a serious issue globally.
Triazine herbicides
Triazine herbicides including atrazine, hexazinone, prometryn, simetryn and ametryn are persistent and accumulate in the food chain, and commonly used as agrochemicals for weed control in industrial farming.
The results have found that aquaculture effluents are also a source of triazine herbicides. As yet, no acute toxic effects on local coral reefs and seagrass beds are observed.
However, it is likely that herbicide contamination has deteriorating effects in the long-term, especially in combination with other stressors such as rising sea surface temperature, which can enhance the sensitivity of organisms to pollutants.
Greenhouse Gas (GHG) emission
It is important to understand aquaculture’s contribution to GHG emissions and how it can be mitigated. We screened the impact of aquaculture activity along its north-eastern coast of Hainan and estimated the impact of eutrophication on oxygen in adjacent waters.
We found that, unsealed ponds acted as CO2 source to the atmosphere due to decomposition of soil organic matter,
accumulating surplus feedings and possibly intrusion of CO2-rich groundwater. In contrast, a sealed and well-managed pond could absorb atmospheric CO2 through conversion into phytoplankton and finally shrimp biomass.
Vibrio
Vibrios are one of the major microbiota of marine ecosystem as well as aquaculture farm. Vibriosis is a most prevalent disease caused by Vibrio spp. and widely responsible for mortality for cultured aquaculture organisms worldwide. Therefore, monitoring of bacteria and, more specifically, Vibrio spp., is of special interest in regions with high amounts of aquaculture, like Hainan.
The results showed that Vibrio abundance is mainly driven by temperature, phosphate and salinity. In addition, it can also be defined by aquaculture operation mode. Classical pond aquaculture decreased Vibrio diversity and significantly reduced the relative abundances of Vibrio compared to coastal reference populations.
Pharmaceutical drugs
Pharmaceuticals found in the coastal include painkillers, medical contrast agents used during x-rays, medicines used for seizures and high blood pressure, and antimicrobials. Pharmaceuticals are not fully metabolized and exit the body through feces or urine. Consumers frequently flush drugs down the toilet as a means of disposal. Pharmaceuticals also enter the river through drug manufacturing plants,healthcare institutions, and agriculture.
Household chemicals
Cosmetic, personal care products and household pesticides are used in huge quantities, as a result of their regular use, they are continuously released into the environment in very large amounts. Many of these products are biologically active and are characterized by persistence and bioaccumulation potential, posing a threat to ecosystem and human health.
Organotin compounds
Organotin compounds, particularly TBT and triphenyltin (TPhT) were intensively used as biocides in antifouling paints in China, until the implementation of the restriction on organotin compounds in antifouling paints since 2011.
However, it only set the maximum permissible limit of total
tin in antifouling paints to 1500 mg per kg dry weight according to the National Environmental Protection Standard of the People’s Republic of China.
The results from our project pointed to antifouling ship paints as origin of the sediment contamination with organotin compounds. Therefore, organotin compounds might act as highly specific molecular indicators for dockyard and shipping activities based on their emission
source specificity and their persistence.
The impact of human activities on Hainan coastal ecosystem
What are seagrasses?
Seagrasses are flowering plants that can live under water, but still require sunlight. They can have many different shapes and sizes. Seagrasses, who belong to the angiosperms, can exist either as a few plants or in dense meadows. Using their strong roots and the extensive underground system, called rhizomes, they anchor
in the sandy and muddy sediments.
They play an important role for the environment by protecting the coast and providing habitat and food for many marine organisms.
What are the threats?
Seagrasses are disappearing worldwide due to:
1) reduced water clarity caused by excess nutrients from sewage, agricultural and urban runoff, aquaculture effluents (e.g. fish and shrimp ponds);
2) discharge of pollutants;
3) physical damage by dredging, boat propellers, anchors, chains,moorings, trawling and shellfish digging;
4) net fishing and blast fishing;
5) natural processes such as storm, flood and tsunam6)i; 6) environmental changes, e.g. in water temperature or salinity
Seagrasses are disappearing in Hainan
Off Hainan, coastal eutrophication from aquaculture effluents has led to an 87% decrease in seagrass biomass and a reduction in species within one decade. The dissolved inorganic nitrogen (DIN) concentration
in the water is a proxy for the intensity of pollution. A threshold for seagrass presence was identified: seagrasses will disappear upon long-term exposure to concentrations >8 µM DIN.
What are Mangroves?
Mangroves are trees or shrubs that grow in intertidal areas of coastal and estuarine environments. They are well adapted to daily inundations with seawater, the low
oxygen content of waterlogged muddy sediments and the salt concentrations that would harm other plants. Mangrove plants developed a special root and salt
filtration system.
Mangrove forests are among the most productive ecosystem in the world. They have valuable ecosystem functions by providing nursery habitat and food for many species. They also act as protective buffer zones that reduce erosion and shield the coasts.
But they have also been under pressure from large-scale conversion into pond aquaculture worldwide.
What are the threats?
The loss of mangroves is mainly caused by anthropogenic activities. Main threats to mangroves are:
1) Conversion of mangrove areas to industrial areas (especially aqua-culture), agricultural land, and human
settlements
2) Overharvesting for firewood, con-struction wood, pulp or charcoal production etc.
3) River changes, tidal barriers, drainage and flood mitigation works
4) Pollutants such as fertilizers, pesticides, oil spills and toxic chemicals
5) Climate change, associated with sea level rise
6) Hurricanes and cyclones
The replacement of Mangrove with aquaculture pond in Hainan
The result showed that overall mangrove loss was 72% (from 3697 ha in 1966 to 1041 ha in 2009), ranging from 63% to virtually 100% loss, depends on the locations. Land cover of aquaculture ponds in the five estuaries increased from 550 ha in1966 to 3944 ha in 2009.55% of the former mangrove area was directly replaced by aquaculture ponds accounting for 76% of the mangrove loss.
An increase in the number of individual mangrove area patches from 230 larger to 2134 smaller patches indicates severe fragmentation of the remaining mangrove areas, likely with adverse consequences for ecosystem functioning.
What are benthic organisms?
Benthic organisms, also called Benthos, are animals and plants that live on, in, or near the bottom of water bodies.
The benthic community is made up of invertebrates, fish, algae, and seagrass. Benthic invertebrates are animals without a backbone and include insects, mollusks
(snails and mussels), worms, and crustaceans (crabs, shrimps). They can be found worldwide in lakes, rivers, estuaries, tidal flats, mangrove forests, and coral reefs. In the oceans they occur from the intertidal zone down to the deep sea.
They are part of many ecosystems worldwide and play an important role as food source, in nutrient recycling,
carbon storage and oxygenation of sediments.
What are the threats?
Benthic communities are strongly affected by environmental conditions, including sediment composition, water quality, salinity, hydrological factors, and by direct human intervention.
The main threats are:
Waste, pesticides, oil pollution, fertilizer impairing the water quality;
Overexploitation, overfishing;
Destruction of their habitats;
Changes in the natural hydrologic regime.
How is the benthos in Hainan?
Effluents from aquaculture and industry and mangrove
loss and fragmentation also affect benthic diversity, community composition and the food web. Gastropod diversity and abundances in the study sites were very
low. Diversity of trees and benthic invertebrates was higher in the larger mangrove area of Bamen Bay than in the small area in Changqi.
No gastropods were found in Quinglan harbor, which is likely a response to high concentrations of organotin compounds used in antifouling paints on ships and to low oxygen concentrations.
What are corals?
Corals are marine invertebrates (animals without a backbone) related to anemone and jellyfish. Corals are found worldwide and are of high importance in many ways, for example as habitat for marine organisms, natural barrier that offers shoreline protection, tourist
attraction or as filter that improves water quality.
What are the threats?
Currently, three quarters of the coral reefs worldwide are
at risk and 50 % of them could be destroyed by 2030. Corals are threatened by various processes and human activities:
1) Extreme weather events such as storms;
2) Climate change, associated with increased water temperature, elevated light levels, and sea level rise;
3) pH changes through ocean acidification;
4) Agricultural and urban runoff increasing nutrients, herbicides, sediment load and turbidity;
5) Organic pollutants, oil spills, marine debris such as microplastics.
How are corals in Hainan?
Our study in Hainan has found that the mean PAH (polycyclicaromatic hydrocarbon) concentrations value in corals were markedly higher than ambient sediments and seawater, demonstrating the bioaccumulation ability of PAHs by corals.
In addition, the results also indicated that PAHs in corals maybe not bioaccumulate from the ambient sediments but through pathways like absorbing from seawater, symbiosis, and feeding. Furthermore, results from comparing three study sites also suggested that land runoff may be an important source of PAH contamination and seawater mobility can strongly impact the PAHs’ distribution in Hainan coral reef ecosystem.
Reference
Xiang, N., et al. (2018). “Occurrence and distribution of Polycyclic aromatic hydrocarbons (PAHs) in seawater, sediments and corals from Hainan Island, China.” Ecotoxicol Environ Saf 152: 8-15.
Thomsen, E., et al. (2020). “The end of resilience: Surpassed nitrogen thresholds in coastal waters led to severe seagrass loss after decades of exposure to aquaculture effluents.” Mar Environ Res 160: 104986.
Möller, L., et al. (2020). “Impact of coastal aquaculture operation systems in Hainan island (China) on the relative abundance and community structure of Vibrio in adjacent coastal systems.” Estuarine, Coastal and Shelf Science 233.
Li, P., et al. (2020). “Unusual tin organics, DDX and PAHs as specific pollutants from dockyard work in an industrialized port area in China.” Chemosphere 243: 125284.
Herbeck, L. S., et al. (2020). “Decadal trends in mangrove and pond aquaculture cover on Hainan (China) since 1966: mangrove loss, fragmentation and associated biogeochemical changes.” Estuarine, Coastal and Shelf Science 233.
Dsikowitzky, L., et al. (2020). “Occurrence and origin of triazine herbicides in a tropical coastal area in China: A potential ecosystem threat.” Estuarine, Coastal and Shelf Science 235.
Copyrights of pictures on this page: Tim Jennerjahn, Inga Nordhaus, Hongwei Zhao, Esther Thomsen, Lucia Herbeck, Lars Möller, Jialin Zhang