Impacts of finfish aquaculture associated with feed type on organic carbon mineralization and benthic nutrient flux in the sediment.

Abstract

Sedimentary geochemistry, rates of organic carbon (Corg) oxidation and benthic nutrient flux (BNF) were measured at two fish farms using raw fish (RF site) and extruded pellets (EP site), to compare the environmental impacts according to feed type on sediment. The fish feed input at the RF site (6.3 kg m-2 mo-1) was relatively lower than at the EP site (9.2 kg m-2 mo-1), but the vertical flux of the suspended particulate matter (SPM) at the RF site (1,323 g SPM m-2 mo-1) was 2 times higher than at the EP site (756 g SPM m-2 mo-1). Benthic Corg oxidation rates such as sediment oxygen demand (SOD: 70.1 mmol O2 m-2 d-1), iron reduction rate (FeRR: 65.2 mmol Fe m-2 d-1), and sulfate reduction rate (SRR: 58.4 mmol S m-2 d-1) were the highest at the RF site compared to the EP site (SOD: 58.4 mmol O2 m-2 d-1, FeRR: 34.7 mmol Fe m-2 d-1, SRR: 21.8 mmol S m-2 d-1), and the control site without fish farming activity (SOD: 14.3 mmol O2 m-2 d-1, FeRR: 24.7 mmol Fe m-2 d-1, SRR: 2.08 mmol S m-2 d-1). NH4+, PO43-, Fe2+, and H2S in the pore water, metabolic products of Corg oxidation, were also the highest at the RF site (11.3 mmol N m-2, 1.96 mmol P m-2, 0.52 mmol Fe m-2, 0.80 mmol S m-2) compare to the EP (8.84 mmol N m-2, 1.04 mmol P m-2, 0.27 mmol Fe m-2, 0.21 mmol S m-2) and control sites (3.24 mmol N m-2, 0.39 mmol P

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m-2, 0.01 mmol Fe m-2, 0.20 mmol S m-2). The dissolved inorganic phosphate (DIP) and nitrogen (DIN) flux at the RF site (1.07 mmol P m-2 d-1, 5.92 mmol N m-2 d-1) were higher than the EP (0.34 mmol P m-2 d-1, 4.82 mmol N m-2 d-1) and control sites (0.04 mmol P m-2 d-1, 0.25 mmol N m-2 d-1). Interestingly, the enhanced DIP flux at the RF site accounted for 240% of the phosphate (P) required for primary production in the water column, which was significantly higher than at the EP site (78%) and control sites (13%). As a result, it was found that the release of P at the RF site may be potential source of P in the coastal ecosystem. Our results suggest that the use of raw fish rather than extruded pellets as fish feed result in high sedimentation rate, which stimulates the metabolic activity in the sediment, and thereby enhancing benthic-pelagic coupling. In conclusion, feed type may be a regulatory factor in the benthic biogeochemical and ecological process in the fish farms.|본 연구에서는 통영 인근 어류 가두리 양식장에서 생사료를 공급하는 지역 (Raw Fish, RF)과 배합사료를 공급하는 지역 (Extruded Pellet, EP)에서 공급하는 사료의 유형에 따른 가두리 양식장 저층 퇴적물 환경 변화를 비교하기 위해 유기물 분해 과정 및 저층 영양염 용출율을 조사했다. 연구 결과, 양식 면적당 사료 공급량은 RF지역(6.3 kg m-2 mo-1)이 EP지역(9.2 kg m-2 mo-1)보다 비교적 적었지만, 침강입자물질 플럭스는 RF 지역(1,323 g SPM m-2 mo-1)이 EP지역(756 g SPM m-2 mo-1)보다 약 2배 높게 나타났다. 퇴적물 산소 소모율(SOD: sediment oxygen demand, 70.1 mmol O2 m-2 d-1), 철 환원율(FeRR: iron reduction rate, 65.2 mmol Fe m-2 d-1), 황산염 환원율(SRR: sulfate reduction rate, 58.4 mmol S m-2 d-1)를 포함한 퇴적물 유기물 분해율은 RF정점이 EP 정점(SOD: 58.4 mmol O2 m-2 d-1, FeRR: 34.7 mmol Fe m-2 d-1, SRR: 21.8 mmol S m-2 d-1) 및 양식 활동이 없는 Control 점점(SOD: 14.3 mmol O2 m-2 d-1, FeRR: 24.7 mmol Fe m-2 d-1, SRR: 2.08 mmol S m-2 d-1)에 비해 가장 높게 나타났다. 유기물 분해산물인 공극수 내 NH4+, PO43-, Fe2+ and HS- 농도도 RF 정점(11.3 mmol N m-2, 1.96 mmol P m-2,0.52 mmol Fe m-2, 0.80 mmol S m-2)이 EP 정점(8.84 mmol N m-2, 1.04 mmol P m-2, 0.27 mmol Fe m-2, 0.21 mmol S m-2) 및 Control 정점(8.84 mmol N m-2, 1.04 mmol P m-2, 0.27 mmol Fe m-2, 0.21mmol S m-2)에 비해 가장 높게 나타났다. 또한, 저층 용존 무기 인(DIP: dissolved inorganic phosphate) 및 용존 무기 질소(DIN: dissolved inorganic nitrogen)도 RF 정점(1.07 mmol P m-2 d-1, 5.92 mmol N m-2 d-1)이 EP 정점(0.34 mmol P m-2 d-1, 4.82 mmol N m-2 d-1) 및 Control 정점(0.04 mmol P m-2 d-1, 0.25 mmol N m-2 d-1) 보다 높게 나타났다. 특히 RF 정점에서의 인 용출은 수층 일차 생산에 필요한 인 요구량에 대해 240%정도에 해당했으며, 이는 EP 정점(78%)과 Control 정점(13%)에 비해 상당히 높았다. 이는 RF정점에서의 인 용출이 수층생태계의 잠재적인 인 공급원으로 작용 할 수 있음을 나타낸다. 따라서, 이와 같은 결과들은 생사료 공급에 의해 발생한 높은 유기물 침강은 퇴적물 내 미생물 생물대사과정을 자극했으며, 이로 인해 benthic-pelagic coupling역시 강하게 나타났다. 따라서, 양식장 내 공급되는 사료의 유형은 양식장 저층 퇴적물 내 생지화학순환 및 생태학적 과정의 조절요인으로 작용 할 것으로 사료된다.; Sedimentary geochemistry, rates of organic carbon (Corg) oxidation and benthic nutrient flux (BNF) were measured at two fish farms using raw fish (RF site) and extruded pellets (EP site), to compare the environmental impacts according to feed type on sediment. The fish feed input at the RF site (6.3 kg m-2 mo-1) was relatively lower than at the EP site (9.2 kg m-2 mo-1), but the vertical flux of the suspended particulate matter (SPM) at the RF site (1,323 g SPM m-2 mo-1) was 2 times higher than at the EP site (756 g SPM m-2 mo-1). Benthic Corg oxidation rates such as sediment oxygen demand (SOD: 70.1 mmol O2 m-2 d-1), iron reduction rate (FeRR: 65.2 mmol Fe m-2 d-1), and sulfate reduction rate (SRR: 58.4 mmol S m-2 d-1) were the highest at the RF site compared to the EP site (SOD: 58.4 mmol O2 m-2 d-1, FeRR: 34.7 mmol Fe m-2 d-1, SRR: 21.8 mmol S m-2 d-1), and the control site without fish farming activity (SOD: 14.3 mmol O2 m-2 d-1, FeRR: 24.7 mmol Fe m-2 d-1, SRR: 2.08 mmol S m-2 d-1). NH4+, PO43-, Fe2+, and H2S in the pore water, metabolic products of Corg oxidation, were also the highest at the RF site (11.3 mmol N m-2, 1.96 mmol P m-2, 0.52 mmol Fe m-2, 0.80 mmol S m-2) compare to the EP (8.84 mmol N m-2, 1.04 mmol P m-2, 0.27 mmol Fe m-2, 0.21 mmol S m-2) and control sites (3.24 mmol N m-2, 0.39 mmol P

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m-2, 0.01 mmol Fe m-2, 0.20 mmol S m-2). The dissolved inorganic phosphate (DIP) and nitrogen (DIN) flux at the RF site (1.07 mmol P m-2 d-1, 5.92 mmol N m-2 d-1) were higher than the EP (0.34 mmol P m-2 d-1, 4.82 mmol N m-2 d-1) and control sites (0.04 mmol P m-2 d-1, 0.25 mmol N m-2 d-1). Interestingly, the enhanced DIP flux at the RF site accounted for 240% of the phosphate (P) required for primary production in the water column, which was significantly higher than at the EP site (78%) and control sites (13%). As a result, it was found that the release of P at the RF site may be potential source of P in the coastal ecosystem. Our results suggest that the use of raw fish rather than extruded pellets as fish feed result in high sedimentation rate, which stimulates the metabolic activity in the sediment, and thereby enhancing benthic-pelagic coupling. In conclusion, feed type may be a regulatory factor in the benthic biogeochemical and ecological process in the fish farms.