Biodeposition Of Filter-feeding Bivalves And Its Ecological Effects In Subtropical Bay

In the last deacdes, there is an increasing in raft farming of filter feeding bivalvesin the worldwide coastal waters. Also, there is a growing awareness of the likelyenvironmental impacts stem from biodeposition lead to extensive and well recognizedstudies.The fouling orgnisms can be extremely abundant in areas of large-scale bivalveculture. Because culturing gear （e.g. buoys, anchors, ropes, and lantern nets） andbivalve shells provide abundant hard substrate for ascidians to colonize. Many foulingspecies such as ascidians are filter-feeders. They may also influence the environmentby producing biodeposition like the bivalves.Many studies on the impacts of bivalves farming on the environment havealready been conducted in China. Howerver, till now, most of the studies wereconducted in temperate zone, north China. And noly a small amounts of Literaturesabout the effects in subtropical area, especially on the field study of warm waterspecies.This experiment aimed to evaluate the effects of bivalves raft culture on thecoastal environment of subtropical bay. The biodeposition rate of bivalvesCrassostrea angulata, Perna viridis, Chlamys nobilis and ascidian Styelaplicata wereinvestigated in Dapeng Cove of the Daya Bay and Shexia Bay of Mirs bay, both weretypical subtropical bivalves’ aquaculture bays in southern China. Experimetalascidians were sampled from culture facilities and shells of the oyster C. angulata orC. nobilis. The contents of organic matters （OM）, total carbon （TC）, total nitrogen（TN）, organic carbon （OC） and organic nitrogen （ON） in biodeposits and in naturalsediments were analyzed and compared. The effects of bivalve biodeposition on oxyzen consuming rate of sediment, thenutrients fluxes rate at the water-sediment interface, and on the organic carbonmineralization were studied in culture area, and also in a non-culture area whichserved as a control. And the organic carbon mineralization are calculated by theexperience value0.85（CO₂/O₂=0.85, release carbon dioxide/oxygen consuming is0.85）The main research results as follows:Firstly, this article gives a review about the impacts of bivalve farming on theecosystem environment. Dense bivalve populations may exert a strong “top-down”control on suspended particulate matter （including phytoplankton, detritus, auto-andheterotrophic picoplankton and microzooplankton） in coastal systems through theirhuge capacity to clear particles from the surrounding waters. In the process offiltration of this material, changes occur in the size spectrum and consistency of thenatural particulate matter deposited as faeces and pseudofaeces, which has also beenshown that large amounts of organic material accumulation on the bottom below thecultures causing strong changes in the physical and chemical characteristics ofsediments. Such as alteration in benthic faunal composition under shellfish culturestructures has ranged from significant change in community composition and biomass,heavy sedimentation of organic material leads to localised enrichment inducingoxygen depletion and altering bacteria and benthic environments. Biodeposition leadsto an increase in microbial activity and an enhancement of nutrient regeneration at thesediment–water interface under the bivalve farming rafts. Benthic mineralizationprocesses will vary depending on suspension filter feeder population and the rate ofmixing of oxygenated water down to the sediment surface.In the experiment bays, the water conditions are different, the yearly averagetemperature,24.94℃of Dapeng Cove, and is higher than24.1℃of Shexia Bay. Thecontent of Chla is5.52μg/L in Dapeng Cove, and higher than the4.30μg/L in ShexiaBay. And the salty in the two aqualcure bay is basically the same,31.19and31.42respectively. Toal particular matters （TPM） are24.06mg/L and25.29mg/L in Dapeng Cove and Shexia Bay, respectively. Particular organic matter （POM） inDapeng is8.98mg/L, and Shexia Bay is8.00mg/L. Phytoplankton and zooplanktonare analyzed and compared between farming and non-farming area. All the fourseasons, no matter in the farming and non-farming area, the Chrysophytax, Pyrroptataand Cyanophyta are the dominant species. In summer, the amounts of phytoplanktonin farming area are higher than the non-farming area. Spring, summer, autumn andwinter, phytoplankton diversity and evenness in the farming area are higher than thenon-farming area. Zooplankton, total biomass with an average of635mg/m³infarming area, and is higher than the non-farming area with an average of492.5mg/m³.The results also indicate that farming area has smaller number of species, zooplanktondiversity and abundance.Biodepsition rate has been determined in situ in Dapeng cove and Shexia bay.The results shows that the biodeposition rate of Chlamys nobilis ranged from60.9to414.7mg/ind/d, with an average of282.8mg/ind/d; Perna viridis ranged from89.9to865.4mg/ind/d, with an average of323.3mg/ind/d; Crassostrea angulata rangedfrom82.3to882.2mg/ind/d, with an average of343.9mg/ind/d; Styelaplicata rangedfrom145.5to1011.8mg/ind/d, with an average of343.9mg/ind/d. And the meancontents of OM, TC, OC, TN and ON in the biodeposits of Chlamys nobilis are51.01，36.22，14.65，5.77and1.95mg/ind/d, respectively; Perna viridis are50.56，29.00，8.83，8.34and2.52mg/ind/d；Crassostrea angulata are50.55，33.77，9.66，9.42and2.51mg/ind/d；Styelaplicata are60.93，34.93，12.53，2.18�'�2.82mg/ind/d, and arehigher than those in natural sediments. There is a positive correlation between bodylength and biodeposition rate, but weak liner negative relevant to the dry weight.Overall, the high biodeposition rate is the highest in summer, and much lower in thereproduction season in spring and colder season in winter, which indicate thatphysiological activity can be the key factor to biodeposition rate. During theexperiment, we also conclude that biodeposition rate increases by the contents ofTPM, POM, Chla, and water temperature. Biodeposition rate of S. plicata are higherthan the other three bivalves.Studies of Nutrients fluxes, oxygen consuming and organic mineralization show that nutrients exchange between water-sediment interfaces are a dynamic equilibrium,which can be vividly described that the nutrient elements in sediment can permeatewater spontaneously and leads to the increasing of nutrients in water. However, whenconcentration of nutrients in water is higher than the nutrients contents in sediment,nutrients also can permeate sediment. The dynamic equilibrium mainly happens in thenon-farming area, which can be explain that the contents of organic matters are lower.Nutrients flux rates (SiO₃^2-, NH₄-N, PO₄³—P, NO₂—N, NO₃--N) in the farming areawith an average of3131.3，1478.0，-7.38，235.5，1430.9μg/m²/h respectively, arehigher than non-farming area with an average of339.8，95.8，95.9，-117.4，-138.9μg/m²/h, respectively. Oxygen consuming rate of farming sediment is74.68mg/m²/h higher than52.48mg/m²/h of non-farming sediment. Mineralization rateorganic carbon of farming sediment is87.62mg/m²/h higher than61.75mg/m²/h ofnon-farming sediment. Although sediment of farming area has a higher mineralizationof organic carbon, organic carbon embedding ratio is as high as81.16%of farmingarea in summer, which is higher than63.99%of the non-farming area.The results indicated that Crassostrea angulata, Perna viridis, Chlamys nobilisand S. plicata have a high biodeposition rateand the bivalves and fouling organismcan be an important contributor to aggregating mineral and organic matter from watercolumn to surface sediment, which may impact the physical, chemical and biologicalconditions of benthic environment. Therefore, when assessing the effects ofsuspended culture of bivalves on the coastal ecosystem, not only the environmentcapacity, but also the farming bivalves, especially the fouling organisms should beconsidered.