Pelagic processes in extensive shrimp po

All rights reserved.Keywords: Primary production; Bacterial production; Water quality; Nutrients; Oxygen production; Oxygen consumption; Bacteria; Extensive shrimp culture 1.. In this pa

Pelagic processes in extensive shrimp ponds of the

Mekong delta, Vietnam Daniel M Alongi a,), Paul Dixon a, Danielle J Johnston a,1,

Ž pH, dissolved O , salinity, temperature or nutrient concentrations as within-site and temporal2 .

variability were large for most parameters The diurnal cycles of physicochemical characteristics were similar to those measured in other unfertilized, low-alkalinity ponds, but these cycles were dampened by intense rainfall Our data indicate that low net primary production, high rates of

respiration, moderate rates of bacterioplankton production, high suspended solid and nutrient concentrations, low and very variable pH and dissolved O2 concentrations, and variations in salinity due to intense rainfall episodes limit shrimp production in these extensive ponds q 1999 Elsevier Science B.V All rights reserved.

Keywords: Primary production; Bacterial production; Water quality; Nutrients; Oxygen production; Oxygen

consumption; Bacteria; Extensive shrimp culture

1 Introduction

Shrimp aquaculture expanded rapidly in Southeast Asia up until the mid-1990s to theextent that land clearing and construction of shrimp ponds in the region is the leadingcause of losses of mangrove forests and other forms of coastal deterioration Worldshrimp production has leveled off in recent years, as many aquaculture farms have either

collapsed or experienced declining yields Lucien-Brun, 1997 Lack of sustainability isdue to many factors, such as poor management, and low quality and overexploitation ofseedstock Knowledge of ecological factors controlling pond production is also crucialfor more effective management and sustainability of aquaculture enterprises

An ability to estimate the production capacity of shrimp ponds relies upon knowledgenot only of the reproductive and physiological capabilities and tolerance limits ofshrimps, but also of system-level processes Processes such as rates of natural and

Sustainability problems and coastal degradation due to intensification of coastalshrimp aquaculture have plagued most Southeast Asian countries, such as the Philip-

pines, Thailand, Taiwan and China Lucien-Brun, 1997 In Vietnam, there has beenintensification of shrimp culture with concomitant loss and damage to coastal habitats.This degradation has been especially severe for mangrove forests in the Ca Mauprovince in the Mekong delta of southern Vietnam This situation has resulted in theestablishment within Ca Mau province of 22 mixed shrimp farming–mangrove forestryenterprises, where both shrimp and mangrove wood are harvested by individual farmers.Shrimp farming in Vietnam is mostly extensive with low average yields Vietnam

accounts for only 4% of world production ; 30,000 mt , but there are ; 200,000 ha inproduction, making Vietnam in area one of the largest shrimp farming nations in

Southeast Asia Rosenberry, 1996

Although there is some indication that the rate of coastal degradation has slowed inthe Mekong delta, current evidence indicates that yields of both shrimp and mangrovewood are declining As in other countries experiencing declining yields, poor shrimpyields in the Mekong delta have been attributed to several factors, such as pollution,poor management and infrastructure, acid sulfate soil, low quality and quantity ofseedstock, uncontrolled coastal development, and overexploitation of wild stocks.The aim of this study was to determine if poor water quality and low phytoplanktonproductivity were responsible for the poor shrimp production in these extensive ponds

In this paper, we compare and contrast rates of pelagic primary productivity, plankton production and growth rates, and bacterioplankton abundance and respiration inrelation to water quality parameters and shrimp yield in separate and mixed, extensiveshrimp farm–mangrove forestry enterprises in the Mekong delta.

bacterio-2 Methods and materials

2.1 Study sites

The study was conducted in shrimp ponds within one enterprise, Tam Giang III

Žlatitude 8.88N, longitude 105.28E , located within the Ca Mau province of the Mekong

delta of southern Vietnam Fig 1 Tam Giang III is an enterprise consisting of

Ž

extensive farms utilizing two different types of farming systems: 1 separate pond

Ž20–30% of farm area and mangrove forest 70–80% areas, and 2 mixed shrimp Ž Ž

on the first spring tide and a 15-day grow-out cycle is initiated, during whichrecruitment and harvesting occurs on consecutive flooding and ebbing tides over3–5-day intervals during spring tides Harvesting is accomplished by placing nets at thefront of the opened sluice gate, while the ponds are drained to ; 20 cm depth Duringthe next neap tide, the sluice gate is closed for 10–12-day grow out There is little waterexchange during this period, although some farmers allow ; 20 cm exchange of water

This recruitment-harvesting cycle is repeated for the rest of the year The dominant

shrimp species cultured are Metapenaeus ensis 48.3% of total harvest , M lysianassa

Ž32.2% and Penaeus indicus 9.7% Average annual shrimp yields are low, ranging Ž

kg ha yr at Pond 22 Clough and Johnston, 1997

2.2 Water quality sampling

Water samples for dissolved and particulate nutrients in each pond were collected

; 5–10 cm below the surface using sterile plastic syringes: one set of triplicate samples

overnight in a LaJolla UV photooxidizer DOC samples 10 ml were acidified with 100

ml HCl to liberate inorganic carbon and kept cool on ice in a container DOC

concentrations were determined on a Shimadzu TOC-5000 analyzer Blanks were runconcurrently on double-distilled water

Samples for particulates were obtained by filtering onto pre-combusted and weighedGFC filters, which were individually air-dried and wrapped in foil after filtering.100–300 ml of pond water were filtered, depending on the suspended load in each pond

Before analysis, the filters were dried 808C for 7 days , weighed and crushed to powderfor carbon and nitrogen analysis on an Antek Model 707C nitrogen analyzer in tandemwith a Beckman carbon analyzer TOC concentrations were obtained by reacting filtersubsamples with hydrochloric acid followed by analysis on a solid-sample Shimadzuanalyzer Accuracy and precision were determined by analysis of certified referencematerials digested with a blank filter; accuracy was within certified limits and precisionwas within 3–5% In October 1996 and November 1997, nutrient samples were taken atthe front, mid and rear of each pond and in the river adjacent to Ponds 12 and 23 InMay 1997, samples were taken at the same pond locations, but no samples were taken inadjacent river water Samples from the ponds were taken when the sluice gates wereclosed or when water exchange was nearly complete In May 1997, particulate sampleswere taken from the three ponds near the sluice gates during both harvesting andsubsequent recruitment stages

Temperature, salinity, dissolved oxygen and pH in pond waters were measured usingHydrolabq DataSonde 3 dataloggers The dataloggers were calibrated as per factory

Fig 1 Map of the location of Ponds 12, 22 and 23 within Enterprise Tam Giang III and the enterprise position

in the Mekong delta of southern Vietnam.

instructions In October 1996, vertical profiles were measured in the ponds and adjacentriver water and a datalogger was deployed for one diel cycle in each pond and in theriver adjacent to Pond 23 Deployment was similar in November 1997, but includingPond 22 In May 1997, dataloggers were deployed for 9 days in Ponds 12 and 23 only

Ž

during grow-out periods In each location, dataloggers were secured in situ at

mid-de-

pth by means of a rope tied onto a pole placed perpendicular to the pond channel

2.3 Primary production and oxygen flux measurements

Primary production was estimated by14C uptake following isotope preparation and

100 ml HCl until counting on return to the laboratory Internal standards were used forquench correction Primary production rates were calculated following Parsons et al

Ž1984 Production on a daily areal basis mg C m Ž y2 dy1.was estimated by dividing bythe water depth and multiplying by 2 assuming that one-half the total daily irradianceoccurs during 1000–1400 h Primary production measurements were made at the front,mid and rear areas of Ponds 12 and 23 and adjacent river locations in May 1997 and atall three ponds and adjacent to Pond 22 in November 1997

Gross primary production and respiration were estimated from changes in oxygenconcentrations in pond and river water incubated in triplicate light and dark bottles

ŽRobertson et al., 1993 At each location, replicate light and dark bottles were gently

filled with surface water and a magnetic stirrer bar placed into each bottle An oxygenprobe was sealed into the top of each bottle containing a specially made cap, then placedinto a magnetic stirrer within a large plastic bin containing ambient water The stirrerswere run using propellers powered by a battery-operated water pump The water in eachbottle was stirred continuously

Each oxygen probe was connected to a TPSq Model WP82 oxygen meter Theelectrodes were calibrated as per factory instructions The experiments were usually run

calculated following the formulae in Parsons et al 1984 The data were converted tocarbon equivalents assuming an RQ and a PQ of 1 Oxygen flux experiments wereconducted only in the front area of each pond and in river water adjacent to Ponds 12and 23 during the three expeditions

2.4 Bacterioplankton measurements

Quadruplicate 20 ml samples for bacterioplankton abundance were taken immediatelybelow the surface at each location using a 50-ml sterile plastic syringe Each sample wasgently decanted into a sterile glass scintillation vial and fixed with 500–1000 ml filtered,buffered formalin Samples were kept cool and dark until return to the laboratory

Bacterioplankton production was estimated via incorporation of tritiated thymidine

ŽBell, 1993 Briefly, 15 ml water samples n s 4 collected in the same manner as for Ž

measurements Triplicate blanks containing 1.1 ml formaldehyde were run concurrently.Initial isotope dilution and time course experiments indicated that a final Tdr concentra-tion of 30 nM and incubation time of 25 min were optimal for the productivitymeasurements After incubation, samples were processed to estimate isotope incorpora-

tion into bacterial DNA as described in Bell 1993 Recovery efficiency was assumed to

be 100%; internal standards were used to correct for quenching.The rate of thymidineincorporation was converted to carbon production assuming a thymidine conversion

and rear areas of Ponds 12 and 23 and adjacent river water; Pond 22 and adjacent riverwater was sampled only in November 1997.

data Level of significance was accepted at P s 0.05.

1996, May 1997 Pond 23 only and November 1997, Enterprise Tam Giang III, Mekong delta, Vietnam

Ž

October 1996 and in November 1997 Table 1 , ranging from 13.8–20.1 Dissolvedoxygen concentrations were normally low in both ponds and the adjacent river water,with most measurements - 3 mg ly 1

May 1997 and C November 1997, Enterprise Tam Giang III, Mekong delta, Vietnam

Pond and site Nutrients

Ž A October 1996

P23 0.13"0.14 1.8"2.2 5.0"4.1 483"25 17.6"8.3 0.69"0.31 P12 0.07"0.02 0.3"0.2 2.2"0.2 317"17 7.1"2.5 0.37"0.06 P22 0.10"0.03 1.8"0.03 14.2"10.3 358"17 8.5"1.3 0.59"0.07 Ri12 0.10"0.02 0.5"0.1 8.5"0.73 317"8 11.3"2.0 0.53"0.05 Ri23 0.13"0.06 0.7"0.1 13.6"6.63 375"17 10.1"0.8 0.58"0.05

Ž B May 1997

P23 0.27"0.19 1.4"2.3 28.4"3.7 617"16 16.4"4.1 0.72"0.14 P12 0.41"0.17 0.7"0.5 19.6"5.9 1025"25 17.8"12.8 0.52"0.20 P22 0.16"0.17 2.9"4.1 17.7"5.7 867"83 24.2"15.5 0.58"0.10

Ž C November 1997

P23 0.05"0.60 0.7"0.2 0.2"0.04 433"50 14.2"1.3 1.1"0.11 P12 0.06"0.03 0.3"0.1 0.2"0.16 542"42 15.8"3.1 1.2"0.2 P22F 0.06"0.03 0.3"0.1 0.2"0.04 517"58 12.6"2.9 1.0"0.3 P22M 0.02"0.01 0.3"0.1 0.2"0.0 542"33 10.4"0.8 1.0"0.4 P22R 0.04"0.01 0.3"0.1 0.0 591"8 9.0"1.6 0.8"0.1 Ri12 0.13"0.10 0.5"0.1 0.2"0.04 600"25 20.1"5.1 1.8"0.2 Ri23 0.07"0.02 0.6"0.04 14.3"2.1 433"42 14.3"2.1 1.0"0.2

Values are summed means "1 SE of replicates from front, middle and rear areas of each pond due to lack of significant location differences.

Abbreviations as in Table 2.

Ž14 .

Fig 4 Rates of primary production C uptake in surface waters in various shrimp pond and adjacent river locations in May and November 1997 Abbreviations: P23F s Pond 23 Front; P23M s Pond 23 Mid; P23R s Pond 23 Rear; R23s river adjacent to Pond 23, etc Note that P23R and P12F were not sampled in May and November, respectively Values are means and vertical bars depictq1 SE Asterisks denote significant differences between seasons within location See Fig 1 for details of locality.

consistent patterns for nutrients between ponds and adjacent river water over time, but

Ž

some patterns emerged: 1 atomic C:N ratios were highest among ponds in November

1997 with the ratio ranging from 10.8–16.2; in the other seasons, the ratio ranged from

3.3 Primary production and respiration

Rates of primary production measured by14C uptake ranged from - 1 to 94 mg C

y 3 y 1 Ž

m d Fig 4 In May, primary production rates in Ponds 12 and 23 were greater

Fig 5 Rates of gross primary production oxygen flux and Pr R ratio bottom graph in various shrimp pond

and river locations, October 1996, May and November 1997 Values are means and vertical bars depict"1 SE.

Numbers above each set of Pr R ratios for each location are means of seasonal ratios See Fig 1 for details of

locality.

than in adjacent river water, but in November 1997, this was true only for Pond 23; rates

of primary production in some, but not all, sites within Ponds 12 and 22 were greater

than in Song Dam Chim Fig 4 Rates of primary production were not significantlydifferent between Ponds 23 and 12 in different seasons; but in November 1997, primaryproduction was greater in Pond 22 than in the other two ponds, except in comparing

production correlated only with salinity r s q0.78

Rates of primary production measured by oxygen flux were significantly greater thanmeasured by 14C uptake Rates of gross primary production ranged among locations

Fig 6 Rates of pelagic respiration oxygen consumption in various shrimp pond and river locations, October

1996, May and November 1997 Values are means and vertical bars depictq1 SE Asterisks denote significant differences between seasons with location See Fig 1 for details of locality.

but equivalent to rates in Pond 12 Seasonal differences within sites were not consistent

ŽFig 6

By difference, rates of net primary production ranged from 0 to 1131 mg C my 3

dy 1

,but were usually zero There were no clear, consistent differences among sites orseasons in rates of net primary production, owing to large variability of oxygen fluxes

among replicates The range of PrR ratios varied widely Fig 5, bottom , ranging from

y0.45 to 3.4 The overall mean PrR of each site was - 1, ranging from 0.2 to 0.9,

with a grand mean of 0.7 Rates of gross primary production and respiration did not

correlate significantly with any other measured variables, but the PrR ratios correlated

ŽFig 7 There were no consistent differences between Ponds 12 and 23 between

Fig 7 Bacterioplankton densities at each shrimp pond and river location with season Values are meanq1 SE Asterisks denote significant differences between seasons within location Abbreviations as in Fig 4 and

H s high tide; L s low tide See Fig 1 for details of locality.