Sydney rock oyster, Saccostrea commercialis (Iredale & Roughley), filtration of shrimp farm effluent: the effects on water quality

Shrimp pond effluent water can contain higher concentrations of dissolved nutrients and suspended particulates than the influent water. Consequently, there are concerns about adverse environmental impacts on coastal waters caused by eutrophication and increased turbidity. One potential method of improving effluent water quality prior to discharge or recirculation is to use bivalves to filter the effluent. In this study, we examined the effects of the Sydney rock oyster, Saccostrea commercialis (Iredale & Roughley), on the water quality of shrimp pond effluent. Effluent from a shrimp farm stocked with Penaeus japonicus (Bate) was pumped directly into 34-L tanks stocked with different densities of oysters. Combinations of live and dead oysters were used to test the effects of three different densities of live oysters (24, 16 and 8 live oysters per tank). The concentrations of total suspended solids, the proportion of organic and inorganic matter, total nitrogen, total phosphorous, chlorophyll a and the total number of bacteria in the pond effluent water were determined before and after filtration by oysters. The oysters significantly reduced the concentration of all the parameters examined, with the highest oyster density having the greatest effect. Shrimp pond effluent contained a higher proportion of inorganic matter (72%) than organic matter (28%). The organic component appeared to be mainly detritus, with chlorophyll a comprising only a minor proportion. Filtration by the high density of oysters reduced the effluent total suspended solids to 49% of the initial level, the bacterial numbers to 58%, total nitrogen to 80% and total phosphorous to 67%. The combined effects of settlement and oyster filtration reduced the concentration of chlorophyll a to 8% of the initial effluent value.     

The effects of different solids and biological filters in intensive pacific white shrimp (Litopenaeus vannamei) production systems

Biofloc systems rely on suspended solids in the water to house microbes that can remove or cycle nitrogenous wastes; however, nitrogen cycling can be inconsistent. In contrast, external biofilters are used in many recirculating systems to provide a more consistent environment for microbes to process nitrogen. Regardless of the biofiltration approach, solids levels must be controlled to prevent issues in shrimp such as gill fouling, low dissolved oxygen levels, and other negative impacts. The purpose of this study was to examine the effects of settling chambers versus foam fractionators for solids filtration and to compare external biofilters to the biofloc approach as biofiltration strategies. Sixteen 1-m³ round, polyethylene tanks were randomly assigned to four treatments, each of which had four replicate tanks. Eight biofloc systems were established: four using settling chambers for solids control (BF-S) and four using foam fractionators (BF-F). The other eight tanks used external biofilters; four had settling chambers (EB-S) and the other four had foam fractionators (EB-F). All 16 systems were stocked with 250 shrimp at an average size of 4.3 g which were grown for 85 days. There were no significant differences in shrimp production between treatments; however, variability was high in biofloc systems. Nitrite levels were significantly lower in systems with fractionators compared to systems with settling chambers. The concentrations of dissolved Na, Mg, Ca, Sr and Ba in the water were significantly reduced in treatments with settling chambers. The results of this study show that filtration choices significantly impact short- and long-term water quality and reusability but may not have much effect on shrimp production in the short-term.     

Waste and particle management in a commercial, partially recirculating trout farm

The present case study, deals with a recently built aquaculture facility using 80–120 L s⁻¹ spring water for trout production. The farm consists of six raceways, discharging in a common outflow channel, leading to a drum filter equipped with 80 μm gauze. About 120 L s⁻¹ of the microscreen effluent is pumped back in the inflow channel of the six raceways. The remaining effluent is oxygenated with pure oxygen in gravity oxygenation units and led to two U-shaped raceways. The farm effluent is finally filtered by a drum filter with 63 μm mesh size. The microscreen backwash sludge is treated in a cone settler, where the sediments are extracted for agricultural manure application. The sedimentation supernatant is further led in a sub surface flow (SSF) constructed wetland prior to discharge.Due to the advanced effluent treatment within the farm, the total farm effect on the receiving effluent is kept to a minimum. The nutrient increase produced by the farm is only 0.03 mg L⁻¹ total phosphorous (TP), 1.09 mg L⁻¹ biological oxygen demand (BOD₅) and, 0.57 mg L⁻¹ total suspended solids (TSS) in the brook. Especially the incorporation of an intermediate microscreen prior to water recirculation, prevents leaching of dissolved nutrients from particulate matter, as large particles are effectively and as fast as possible removed from the water flow.At the pumping station, needed for water recirculation, the particle size distribution (PSD) was monitored with the previous microscreen in use and by-passed. When the screen was by-passed a significant crushing effect on PSD through pumping action was found. Through the removal of large particles, the crushing effect of the pumping station on the particles is prevented, as revealed by particle size distribution (PSD) measurement. Thus, leaching of dissolved nutrients is prevented twice.In consequence, the farm configuration can be recommended as an effective possibility for intensive trout production at sites with a small freshwater source and stringent effluent thresholds, even with the unexpected low treatment efficiencies measured for the microscreens. Both drum filters showed relative low treatment efficiencies of 33–53% for total suspended solids, respectively, while an efficiency of 70% should be expected from the measured PSD. With this impact, the farm still emitted a low nutrient amount, especially due to the highly effective offline microscreen backwash sludge treatment, where the SSF wetland efficiently reduced dissolved and particulate nutrients as nitrite (NO₂-N), nitrate (NO₃-N) and TSS. Thus this SSF wetland application might be suitable as a denitrification step in a closed recirculating trout farm.     

Water quality and waste production in a recirculating trout-culture system with feeding of a higher-energy or a lower-energy diet

Higher-energy fish feeds can reduce waste discharges and might also improve water quality in recirculating fish-culture systems. A higher-energy diet, Zeigler Salmon High Energy feed (HE; 45% protein, 20% fat, 17.4 MJ digestible energy kg^-1) and a lower-energy diet, Zeigler Hi-Fat Trout Grower (LE; 38% protein, 12% fat, 14.6 MJ digestible energy kg ^-1) were fed ad libitum at different times to rainbow trout, Oncorhynchus mykiss (Walbaum), in a semi-closed recirculating culture system by means of demand feeders. The system contained two 10-m³ fish-culture tanks, each with a downstream microscreen (80 μn) filter. Feeding rates per day and per unit biomass were not significantly different between diets. In general, use of HE was associated with higher levels of total ammonia nitrogen (TAN) and NO₂-N, lower BOD₅ and total suspended solids (TSS), and lower effluent releases of suspended solids per unit feed, NO₃-N per unit feed, and dissolved phosphorus per unit feed. Although total effluent P per unit feed or P fed did not differ significantly between diets, HE had significantly more of the total effluent P in the settleable solids, 85% vs. 76%. Differences in water quality in the system were probably not of great importance with respect to fish health.     

Use of fluidized bed biofilter and immobilized Rhodopseudomonas palustris for ammonia removal and fish health maintenance in a recirculation aquaculture system

Intensive recirculating aquaculture relies on biofilters to sustain satisfactory water quality in the system. Fluidized bed and immobilized cell technologies were used to remove ammonia from the water and maintain fish health. A high‐rate nitrifying fluidized bed biofilter combined with valveless filter was designed for use in a recirculation aquaculture system (RAS). The suspended solids produced during fish culture could automatically be removed using a valveless filter. Natural porosity with fitting proportion, steady fluidization and expanding rate was chosen as the fluidized carrier. The technology of bacterial separation and cultivation was used. The immobilized Rhodopseudomonas palustris (R. palustris) produced through a biotechnologically embedding medium is suitable for fish and could help prevent diseases. Nitrification was promoted through the selective rearing of nitrobacteria in a fluidized bed biofilter. Water quality was improved using fluidized bed biofilter and immobilized R. palustris in the RAS. In addition, the proposed system was able to reduce costs. Maximum fish load was 45 ± 3 kg m^?3 in the closed recirculating water fish culture system, and water use was reduced by 80–90%. The total ammonia nitrogen removal rate of the technology was 80–95%, and nitrite N removal rate was above 80%.     

Performance of sequencing microbead biofilters in a recirculating aquaculture system

Biological filtration, or biofiltration, is the key technology in recirculating aquaculture systems. Sequencing microbead biofilters, in which the media maintains a continuous up-and-down movement, are based on traditional microbead filters but offer superior filtration properties. The performance characteristics of a sequencing microbead biofilter installed in a recirculating aquaculture system for rearing Barcoo perch at 29 ± 1 °C were examined. The total ammonia-nitrogen (TAN) concentrations and the nitrite-nitrogen concentrations during a 52-day culture period were maintained blow 1.6 mg/L and 0.9 mg/L. In order to ensure efficient biofiltration, the optimal actual application of hydraulic retention time was determined to be approximately 3–5 min. The water flow produced by the reciprocating motion of the media served to wash away suspended solids, ensuring the occurrence of optimal nitrification processes. Additionally, the reciprocating motion of the media enhanced ammonia treatment efficiency significantly by improving the transport of nutrients and nitrification activity. Compared to a static situation the ammonia removal rate increased by 27% based on the application of up-and-down reciprocating movement. The biofilm on the microbead forms as a compact, complex, and homogeneous structure, consisting of numerous microscopic thin sheets. Additionally, a multitude of pores, interstitial voids, and vertical channels were widely observed to convey obviously advantageous properties in support of fluid passage, thus enhancing mass transfer and ultimately contributing to biofiltration effectiveness. The optimum biofilm thickness for providing efficient biofiltration was determined to be approximately 70 μm for this filter.     

The use of high rate algal ponds for the treatment of marine effluent from a recirculating fish rearing system

A high rate algal pond (HRAP) system was used to treat effluent from a recirculating sea water aquaculture system in southern France. Dicentrarchus labrax L. were farmed at a high density, with effluents containing an average of 10 mg L^?1 dissolved inorganic nitrogen (DIN) and 1.3 mg L^?1 reactive phosphorus (RP). On a yearly basis, the algal pond removed 59% of the dissolved nitrogen and 56% of the phosphorus input, which was converted into 3.3 kg DW m^?2 algae. Green macroalgae were dominant throughout the year and the algal biomass mirrored the seasonal changes in daily irradiance and temperature. This first year study supports the possibility of treating marine aquaculture wastes using HPAPs, although conditions will have to be found to mitigate the strong influence of climate on the algal community during winter. During the more temperate season, only 150 m² of treatment ponds would be necessary to remove the nutrients produced by 1 ton of fish. Treated water was characterized by a high pH, elevated levels of dissolved oxygen (midday value) and low concentrations of nutrients and suspended solids. The absence of toxic phytoplankton meant that the water could be recycled through the farm tanks.     

Experimental Evaluation of the Halophyte,Salicornia virginica,for Biomitigation of Dissolved Nutrients in Effluent from a Recirculating Aquaculture System for Marine Finfish

The ability of the halophyte, Salicornia virginica, planted in drainage lysimeters to biomitigate dissolved nutrients in effluent from a recirculating aquaculture system (RAS) for marine finfish was evaluated. Seawater effluent from a RAS producing black sea bass, Centropristis striata (filtered to reduce total suspended solids), was used as irrigant. Plant growth and dissolved N and P removal were determined as a function of leachate fraction (LF%) – that is, proportion of irrigant that leaches from the plant‐substrate lysimeter. Lysimeters were irrigated weekly to produce 30, 40, and 50% LF. A control (unplanted) lysimeter was included at the 30% LF. Plant growth was excellent in all LF% treatments until Day 141 when salt buildup in the lysimeter substrate inhibited nutrient uptake. Salt accumulation was mitigated at higher LF%, so that plant biomass and net removal (μg) of dissolved N and P by the p‐s lysimeter remained higher (P < 0.05) at the 40 and 50% than at the 30% LF. On Day 141, percent removal efficiency at the 50% LF was 79.2% for inorganic N and 73.9% for total phosphorus. Through Day 355, substrate salinity was minimized and plant biomass and nutrient removal were maximized at the 50% LF. S. virginica is an effective biofilter for dissolved nutrients in effluent from an RAS for marine finfish.     

Performance characteristics of fluidised bed biofilters in a novel laboratory-scale recirculation system for rainbow trout: nitrification rates, oxygen consumption and sludge collection

A laboratory-scale recirculating aquaculture system for fluidised bed biofilter evaluation was engineered. The design included all components found in typical full-scale commercial production systems. The system included two identical units each with oxygenation, UV treatment, cooling, biofiltration and a particulates separation device. Water from the two systems was mixed in a degassing unit. A 1 month test period after biofilter maturation revealed stable concentrations of total ammonia nitrogen (TAN), nitrite and nitrate within the system. Mean nitrification rate was 0.27 and 0.21 g TAN m⁻² day⁻¹. Oxygen consumption in the biofilters ranged between 56 and 64% due to nitrifying activity. Mass balances on nitrogen indicated that 48%, added via the feed, was converted to nitrate within the system, with 6% of the added nitrogen being found in the sludge. The remaining 43% was either used during fish growth, left the system, as organic nitrogenous compounds (or unidentified nitrogenous compounds), via the outlet, or was lost to the atmosphere. At least 61% of the nitrate produced was generated by the biofilters. The system proved to be an exceptional set-up for evaluation of the performance of fluidised bed biofilters, allowing both pre- and post-filter measurements of various water quality criteria.     

The use of mangrove wetland as a biofilter to treat shrimp pond effluents: preliminary results of an experiment on the Caribbean coast of Colombia

The potential benefit of integrating mangrove and shrimp farms to protect ponds against erosion, to enhance the productivity of supply water and also to treat pond effluents has been pointed out previously. Agrosoledad, a 286‐ha shrimp farm located on the Caribbean coast of Colombia, was constructed behind a 1‐km‐wide mangrove area. Farm effluents are partially recirculated through a 120‐ha mangrove wetland used as a biofilter. A 3‐month study compared the concentrations of suspended solids and inorganic nutrients in the supply canal, the pond drainage and the biofilter. Suspended solids increased in pond drainage compared with supply water, but they were drastically reduced in the biofilter. In contrast, dissolved inorganic nitrogen and phosphorus concentrations were not different in supply water and pond drainage, but they increased in the biofilter because of the presence of a large marine bird community. Additionally, a significant decrease in dissolved oxygen and pH was observed in the biofilter. The study demonstrated the efficiency of the system to eliminate suspended solids from the effluent. However, nutrient dynamics showed that the possible use of mangrove wetlands as biofilters for effluent treatment will be less predictable than expected.     

Biological filters in aquaculture: Trends and research directions for freshwater and marine applications

Factors such as limitations in water quality and quantity, cost of land, limitations on water discharges, environmental impacts and diseases, are driving the aquaculture industry toward more intensive practices. This will force producers to adopt environmentally friendlier technologies. Recirculating systems, with a biofilter as the most prominent characteristic, treat internally the water contaminated with dissolved organics and ammonia and reduce the amount of water use and discharge from aquaculture operations. This paper reviews the implications of the changing use of recirculating aquaculture systems (RAS) on biofiltration research for freshwater and marine operations. Demand for cost effective biofilters will increase with the expansion of recirculating systems, both as a complement and replacement of traditional ponds. For freshwater aquaculture, emphasis should be placed in cost competitiveness, low head operations, intensification of ponds with RAS biofiltration and the evaluation of suspended growth systems. In the marine systems, an increase in demand of oligotrophic and ultraoligotrophic systems is expected, particularly in the nursery systems. Sizing and cost efficiency of biofilters for nursery operations should be addressed. Problems in marine biofilter acclimation appear to justify the development of new acclimation procedures. Biosecurity concerns, land cost and storm threats will drive nursery systems inland, where saltwater supply and disposal will force an increased water reuse. Denitrification strategies will need to be redefined and optimized for the marine nursery environment.     

Design and performance of a recirculating aquaculture system for oyster larval culture

The major objective of this study was to introduce a newly designed recirculating aquaculture system (RAS) for oyster (Crassostrea angulata) larval culture. The system includes a culture tank, a suspended circular inlet‐pipe, an upwelling aeration pipe, combined “banjo” sieves and a bioreactor chamber containing microalgae life keeping installation. The system was designed to resolve three problems: (i) stranding of larvae caused by water level changes and aeration, (ii) physical clogging of the screens and also (iii) deterioration of diet microalgae. The culture tank, “banjo” sieve size, water flow rate and light intensity for maintaining microalgae activity were all designed according to the pattern of larval movement and feeding behaviour. Results of this study showed the best average SGR for larval length was 6.36%/d (9.5 μm/d) and survival rate was 80%, with initial rearing density of 50 larvae/ml, indicating the problems above were fully resolved. Consequently, the system is fit for larval culture in mass production of oysters.     

Oyster (<Emphasis Type="Italic">Crassostrea virginica</Emphasis>) gardening program for restoration in Delaware’s Inland Bays,USA

Eastern oyster, Crassostrea virginica is a keystone species in many estuarine bays. They clean the water by filtering out suspended particulates from the water column, while their reefs serve as valuable habitat for many ecologically and economically important macrofaunal species. However, with the ever-increasing development along our coastlines, the oyster population throughout the Mid-Atlantic region has plummeted as a result of overharvesting, extreme eutrophication, and disease. In response to this detrimental decline, many conservation organizations in the area have developed community-involvement programs commonly referred to as ‘oyster gardening’ to help restore the oyster population, while instilling within the community a strong sense of stewardship for their bays. Although the oyster gardening program in Delaware is relatively new, its popularity has grown tremendously. It is the hope of many Delaware residents that the reintegration of oysters into the Inland Bays will help to return their bays to the state of health in which they once were. Despite the expansion, further development of designated reef area is necessary to ensure the successful proliferation of C. virginica in Delaware’s Inland Bays.     

Effects of artificial substrate and night‐time aeration on the water quality in Macrobrachium amazonicum (Heller 1862) pond culture

The effects of artificial substrate and night‐time aeration on the culture of Macrobrachium amazonicum were evaluated in 12 ponds stocked with 45 prawns m^?2. A completely randomized design in 2 × 2 factorial scheme with three replicates was used. The combination of factors resulted in four treatments: with substrate and aeration (SA), with substrate and without aeration (SWA), without substrate and with aeration (WSA) and without substrate and aeration (WSWA). The presence of substrate in SA and SWA treatments reduced suspended particles (seston) by ~17.3% and P‐orthophosphate by ~50%. The use of aerator (WSA and SA treatments) significantly (P < 0.05) increased the concentration of dissolved oxygen, suspended particles and nutrients in the pond water. These results indicate that the effect of substrate on turbidity and total suspended solids (TSS) values is opposite to the effect of the aerator. The aerators in semi‐intensive grow‐out M. amazonicum farming lower water quality because they increased the amount of detritus and nutrients in the pond water. On the other hand, the use of artificial substrate reduces turbidity values, chlorophyll a, TSS and P‐orthophosphate concentrations. Therefore, the combination of substrate addition and night‐time aeration is not interesting because they have opposite effects.     

Screening and Evaluation of Alum and Polymer Combinations as Coagulation/Flocculation Aids to Treat Effluents from Intensive Aquaculture Systems

As the application of intensive aquaculture systems continues to grow worldwide, so does the need for environmentally sound waste management and waste disposal techniques. The use of coagulants and flocculants to assist in removing suspended solids has long been a standard in the waste treatment field. Recently, The Conservation Funds Freshwater Institute has continued its research into coagulation/flocculation aids by examining the performance of alum coupled with various commercially available polymers. Alum is efficient in sequestering phosphorus through chemical precipitation and coagulation of fine solids through charge neutralization. Synthetic polymers are efficient in flocculating small particles together but do not efficiently remove dissolved phosphorus. The specific intention of this work was to use the qualities that distinguish both the alum and the polymer individually and combine the two to optimize wastewater treatment for the removal of both suspended solids and phosphorus. The alum/polymer combinations were first screened to determine which polymers worked best with our waste and with the alum. Once the screening was completed, the six best performing combinations were further evaluated with triplicate tests in a jar test apparatus to determine a standard optimal dosage based on phosphorus and suspended solids removal. Using a combination of alum/polymer, the effluent total suspended solids (TSS) removal rate was close to 99%, with final TSS values ranging from 4 to 20 mg/L. Reactive phosphorus was reduced by 92 to 99% to as low as 0.16 mg/L‐P. Finally, total phosphorus was also significantly reduced (98%), with treated effluent concentrations from 0.9 to 3.0 mg/L‐P. Although not intended for nitrogen removal, total ammonia‐nitrogen (TAN), nitrite‐nitrogen, nitrate‐nitrogen, and total nitrogen in the effluent were reduced on average by 64, 50, 68, and 87%, respectively. Removal rates for both 5 d carbonaceous biochemical oxygen demand (CBOD₅) and chemical oxygen demand (COD) were also significant, with an average value of 97.3 and 96.4%.     

Effects of different feed management treatments on water quality for Pacific white shrimp Litopenaeus vannamei

Increasing feeding rates may provide an increase in production, thus nutrients such as nitrogen, phosphorus and organic matter will also increase. These nutrients promote a greater oxygen demand and concentrations of toxic metabolites which can lead to frequent problems with low dissolved oxygen and an abundance of blue‐green algae. Four feed management practices were evaluated among sixteen 0.1 ha ponds culturing Pacific white shrimp (Litopenaeus vannamei). Feeding treatments included hand feeding using the Standard Feeding Protocol (SFP), SFP plus 15% from 8 to 16 weeks, an automatic‐solar timer which fed SFP+15%, and an AQ1 acoustic demand feeder allowing up to 12 kg/day·pond based on shrimps feeding response. Samples were analysed at weeks 0, 4 and 8–16 for the following parameters: chlorophyll a, total ammonia nitrogen, nitrite–nitrogen, nitrate–nitrogen, total nitrogen, total phosphorus, soluble reactive phosphorus, total suspended solids, total suspended volatile solids, turbidity, conductivity, salinity and biological oxygen demand. Samples were collected and shipped overnight to Auburn, Alabama for off‐site analysis. On‐site water quality was also obtained at the farm. The AQ1 acoustic demand feeder produced the most shrimp with a yield of 4,568 kg/ha; however, the AQ1 also had the highest total ammonia nitrogen and nitrite–nitrogen levels late in the growing season. The AQ1 feeder may be a viable, reduced labour and cost alternative for the shrimp commercial industry; however, such technologies must also be matched to the ability of the production system to process nutrients.     

Evaluation of Two Textiles with or without Polymer Addition for Dewatering Effluent from an Intensive Biofloc Production System

Intensive, recirculating aquaculture systems create concentrated wastes high in solid content. Geotextile has successfully dewatered aquaculture effluent; however, burlap, made from natural plant fiber, may provide similar filtering capabilities at a lower cost. The trial was designed as a 2 × 2 factorial to evaluate burlap bags and geotextile bags with or without polymer addition for dewatering Nile tilapia, Oreochromis niloticus, effluent from an intensive biofloc production system. There were no significant interactions (P > 0.05) between the main effects on the removal efficiency of total suspended solids (TSS) concentration. There were no significant differences (P > 0.05) in the main effect of textile; however, there were significant differences (P≤ 0.001) in the main effect of polymer on the removal efficiency of TSS concentration from effluent. Overall, TSS removal efficiency in textile‐only treatments was 81%, whereas textile treatments in combination with polymer removed 98%. Partial budget analysis indicated that the cost per kilogram of solids (dry weight) removed from untreated effluent was US$1.52, 1.51, 0.16, and 0.14 for the geotextile with polymer (GP), geotextile without polymer (GNP), burlap with polymer (BP), and burlap without polymer (BNP) treatments, respectively. The BP could provide an effective treatment process for removing TSS in discharged effluent.     

Growth,morphometrics and nutrient content of farmed eastern oysters,Crassostrea virginica (Gmelin), in New Hampshire,USA

When harvested, oysters represent a removal from the ecosystem of nutrients such as nitrogen (N) and carbon (C). A number of factors potentially affect nutrient content, but a quantitative understanding across the geographical range of the eastern oysters is lacking. This study was designed to quantify the relationships among various metrics of farmed eastern oysters near its northern geographical range focusing on nutrient content. Hatchery‐reared oysters were deployed in polyethylene bags at six sites, and were measured on multiple occasions from 2010 to 2012. A quadratic polynomial fit to the combined datasets for shell height indicated that on average a ‘cocktail’ size oyster (63 mm shell height) would be reached after 2 year, and ‘regular’ size (76 mm) would require 3 year. There were significant differences in growth rates and oyster nutrient content among the sites; means for %N in soft tissue ranged from 6.9 to 8.6, and 0.07 to 0.18 in shell. Per cent N in soft tissue and shell were highest at two sites at the mouths of rivers with elevated dissolved inorganic N concentrations in the water. Grand means (all sites, seasons and years combined) of soft tissue N and C for regular size oysters were 7.3% and 38.5%, respectively; and for shell N and C were 0.13% and 12.0% respectively. Our study extends the range of data on nutrient content of the eastern oyster to northern New England, and indicates that oyster size, seasonality, and nutrient concentration in ambient water potentially affect %N and %C content of oysters.     

Short term effects of a commercial eastern oyster nursery upon nutrient and plankton dynamics of a coastal embayment: observations from mesocosm experiments

Two mesocosm experiments were conducted to study the interaction of a commercial oyster‐nursery system with the surrounding embayment, specifically, nutrient and plankton dynamics. The nursery was stocked with 80–200 L of oysters, sized 3–5 mm. Ambient water and the water passed through the oyster nursery were incubated in mesocosms, and nutrients and plankton were monitored over the course of 27‐h incubations. The first experiment conducted in July 2009 captured a phytoplankton community dominated by Peridinium quinquecorne, a rare dinoflagellate not previously reported in the northeastern USA. Dissolved nitrogen compounds in all mesocosms were depleted within the first 6 h. The second experiment was conducted in September 2009, when a more typical and diverse phytoplankton community was present. Unlike the first experiment, Nitrate plus nitrite in the mesocosms was not depleted. Both experiments indicated little difference between the water passed through the oyster nursery and the ambient water in terms of subsequent nutrient and plankton dynamics, indicating little ‘legacy effect’ of the oyster nursery upon nutrients and plankton in this embayment. This conclusion was reached under the specific operating conditions typical for this nursery system, as chosen by the commercial partner in the study; changes in oyster stocking or flow rate are likely to change the results.     

Prototype Testing of the Appurtenance for Settleable Solids In-raceway SeparaTion (ASSIST)

The most common type of rearing unit for rainbow trout produced in the US is the flow-through raceway. A water treatment structure, which can be readily retrofitted into existing raceways, was designed to create rotational currents to aid in the concentration and collection of particulate waste material in situ. The designed structure, referred to as the Appurtenance for Settleable Solids In-raceway SeparaTion (ASSIST), incorporated a drain line which diverts a small fraction of the raceway flow to continuously remove the uneaten feed and fecal material from the collection area located at the effluent end of the raceway. Continuous removal of the collected particulates minimizes the leaching of soluble material from the solids into the water column. Based on scale model tests using analogous waste material, the ASSIST is expected to remove 40–50% of the settleable solids generated in commercial rainbow trout aquaculture while diverting 5–10% of the raceway flow.