Secchi Disk Visibility Research Articles

Are artificial reefs comparable to neighbouring natural rocky areas? A mollusc case study in the Gulf of Castellammare (NW Sicily)

Mollusc assemblages of three concrete artificial reefs (ARs) in the Gulf of Castellammare were compared with those of two nearby natural reefs (NRs). The reefs were located in areas characterized by different degrees of water transparency, with average annual Secchi disk visibility ranging from clear (>20 m) to turbid waters (<6 m). In spring 1995, 28 samples of 400 cm 2 were scraped off the reefs at depths of 16-22 m (20 from ARs; 8 from NRs), which yielded a total of 116 species and 1084 specimens of molluscs. The assemblage derived from a NR in clear water showed the highest diversity, species richness, and evenness, while the highest number of specimens and dominance values were observed at an AR in turbid water. The lowest values of diversity and species richness were found at ARs in turbid water and the lowest number of specimens at a NR in turbid water. Comparison of the five reefs showed significant differences in average number of species and in diversity values, while differences in the number of specimens were not significant. Factorial correspondence analysis showed a pattern that was strongly polarized along the first axis by the NR in clear water and by the AR in turbid water. The most distinctive feature was the dominance of Bittium latreillii at AR sites. Three years after deployment, mollusc assemblages of ARs in the Gulf remain entities that are distinct from those of nearby natural reefs. However, the potential of the assemblages in terms of density of individuals is comparable to that of natural reefs. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

The effect of periphyton and supplemental feeding on the production of the indigenous carps Tor khudree and Labeo fimbriatus

Two experiments were conducted, one with the herbivorous mahseer Tor khudree and another with the fringe-lipped carp Labeo fimbriatus to study the effect of feeding and periphyton on their growth and production. Twelve tanks (25 m 2) with mud bottoms and 0 (control), 98 (low density) or 196 (high density) bamboo poles tank −1 were used. Mahseer (initial wt. 3.5 g) and fringe-lipped carp (initial wt. 0.73 g) stocked at 25 fish tank −1 were reared for 90 and 60 days, respectively. Fish in half of the tanks received a pelleted feed (35% protein) at a ration of 5% body weight day −1. Water was monitored daily for dissolved oxygen, pH, temperature, and Secchi disc visibility, and weekly for ammonia, nitrate, phosphate, and alkalinity. Periphyton dry matter, ash, and chlorophyll content were quantified fortnightly. Periphyton biomass, measured as total pigment (chlorophyll- a+pheophytin), decreased with time in both experiments indicating effective grazing by the fish. In the control treatment (without supplemental feed and substrates), mahseer grew to a final weight of 30.8±0.9 g and fringe-lipped carp to 19.1±1.1 g (mean±S.D.). Supplemental feeding alone resulted in higher final mean weight of mahseer and fringe-lipped carp of 38.3±0.6 and 22.9±1.2 g, respectively. The provision of substrates resulted in final mean weights of mahseer of 36.0±5.7 and 36.1±1.3 while that of fringe-lipped carp were 22.8±1.2 and 22.5±1.9 (low and high density, respectively). Net production of mahseer with substrates was 41% and 51% higher, and that of fringe-lipped carp was 43% and 75% higher than in the control (low and high density, respectively). The combination of substrates and supplemental feed resulted in mean final weights of mahseer of 40.3±0.4 and 39.7±1.2, and of fringe-lipped carp of 23.6±1.4 and 25.5±0.1 (low and high substrate density, respectively). Production of mahseer with a combination of substrates and feeding was 71% and 54% higher, and of fringe-lipped carp, it was 85% and 87% higher than in the control (low and high density, respectively). It is concluded that the provision of substrates can reduce the need for artificial feed and can be an alternative to feeding in the culture of herbivorous fish.

Organic matter sedimentation and resuspension in tilapia ( Oreochromis niloticus) ponds during a production cycle

The rates of sedimentation and resuspension of organic carbon and total nitrogen were measured in earthen fishponds, based on nutrient input, water quality parameters and fish size and biomass. Material collected in sediment traps and soil samples were analyzed for organic carbon, total nitrogen, iron and aluminum concentrations. A dilution analysis method was used to differentiate between sedimented and resuspended particles. The rates of sedimentation and resuspension estimated from total solids increased during the experiment, ranging from 49.8 to 218.1 g/m 2 per day in the case of sedimentation and from 39.2 to 160.0 g/m 2 per day in the case of resuspension. Although fish weight increased through time, the relative resuspension did not change significantly, ranging from 42 to 47% of the total collected material. Total solids sedimentation and resuspension rates were highly correlated ( P≤0.01) to fish weight and biomass, chlorophyll- a, total suspended solids, total feed input and Secchi disk visibility.

TURBIDITY SUSPENI)ED SEDIMENT, AND WATER CLARITY: A REVIEW1

ABSTRACT: Suspended sediment causes a range of environmental damage, including benthic smothering, irritation of fish gills, and transport of sorbed contaminants. Much of the impact, while sediment remains suspended, is related to its light attenuation, which reduces visual range in water and light availability for photosynthesis. Thus measurement of the optical attributes of suspended matter in many instances is more relevant than measurement of its mass concentration. Nephelometric turbidity, an index of light scattering by suspended particles, has been widely used as a simple, cheap, instrumental surrogate for suspended sediment, that also relates more directly than mass concentration to optical effects of suspended matter. However, turbidity is only a relative measure of scattering (versus arbitrary standards) that has no intrinsic environmental relevance until calibrated to a ‘proper’ scientific quantity. Visual clarity (measured as Secchi or black disc visibility) is a preferred optical quantity with immediate environmental relevance to aesthetics, contact recreation, and fish habitat. Contrary to common perception, visual clarity measurement is not particularly subjective and is more precise than turbidity measurement. Black disc visibility is inter‐convertible with beam attenuation, a fundamental optical quantity that can be monitored continuously by beam transmissometry. Visual clarity or beam attenuation should supplant nephelometric turbidity in many water quality applications, including environmental standards.

Dry‐Tilling of Pond Bottoms and Calcium Sulfate Treatment for Water Quality Improvement

AbstractThree different pond bottom treatments were evaluated in 12 earthen ponds. Bottoms of four ponds on the Auburn University Fisheries Research Unit, Auburn, Alabama, were dried for 1 mo and tilled with a roto‐tiller (dry‐till treatment). Four other ponds were dried and tilled, and after filling with water, enough gypsum (calcium sulfate) was applied to give a total hardness of about 200 mg/L. Gypsum was reapplied as needed to maintain the hardness (dry‐till with gypsum treatment). Four ponds were not subjected to bottom drying, tilling or gypsum treatment (controls). Channel catfish Ictalurus punctatus fingerings were stocked at 15,000/ha. Selected water quality variables were measured at 1‐ to 2‐wk intervals during the growing season. Concentrations of most variables increased over time because feeding rate was increased progressively as fish grew. Compared to the controls, both treatments had lower concentrations of total phosphorus and soluble reactive phosphorus, and higher concentrations of dissolved oxygen (P &lt; 0.05). In addition, ponds of the dry‐till with gypsum treatment had in addition lower concentrations (P &lt; 0.05) of chlorophyll a, chemical oxygen demand, gross primary productivity, and total alkalinity than control ponds. The reduction in chlorophyll a concentration suggested less phytoplankton in gypsum‐treated ponds, and this effect was probably related to lower phosphorus availability because of calcium phosphate formation. Secchi disk visibility, total suspended solids concentrations, and turbidity did not differ significantly among the treatments (P &lt; 0.05). Total fish production and survival rate did not differ significantly (P &lt; 0.05) among the treatments. These findings suggest that water quality improvement can be achieved by drying and tilling pond bottoms between crops, and benefits possibly may be increased by treating low hardness waters with gypsum.

Effects of bottom soil management practices on water quality improvement in channel catfish Ictalurus punctatus ponds

The bottoms of four ponds on the Auburn University Fisheries Research Unit, Auburn, AL, were dried for 1 month and tilled with a roto-tiller (dry-till treatment). Four other ponds were dried and tilled, sodium nitrate (200 kg ha −1) was applied to oxidize bottom soil, and soils were tilled again to mix bottom soil with sodium nitrate (dry-till with sodium nitrate treatment). Four ponds were not subjected to bottom drying, tilling or sodium nitrate treatment (controls). Channel catfish ( Ictalurus punctatus) fingerings were stocked at 15,000 ha −1. Selected water quality variables were measured at 1–2-week intervals during the growing season. Concentrations of most variables increased over time because feeding rates were progressively increased as fish grew. Compared to controls, both treatments had lower concentrations of soluble reactive phosphorus, nitrate nitrogen, nitrite nitrogen, total ammonia nitrogen, total suspended solids and turbidity, and higher values of pH, Secchi disk visibility, total alkalinity, total hardness, and calcium hardness ( P<0.01). Ponds of the dry-till treatment had in addition lower concentrations ( P<0.01) of total phosphorus and total nitrogen than control ponds. Concentrations of dissolved oxygen and chemical oxygen demand did not differ significantly among the treatments ( P>0.05). Compared to the control, both treatments had lower concentration of chlorophyll a from June to July ( P=0.004), but controls had lower concentrations of chlorophyll a from August to September than both treatments ( P<0.001). Organic carbon, total phosphorus, and dry pH in soil samples did not differ among the treatments ( P>0.05). The findings suggest that water quality improvement can be achieved by drying and tilling pond bottoms between crops. Applying sodium nitrate to dry, tilled pond bottoms did not increase the degree of water quality improvement.

The relationship between the spatial distribution of fish, zooplankton and other environmental parameters in the Solina reservoir, Poland

The hydroacoustic estimation of fish abundance and distribution in the Solina reservoir was related to other biological and hydrological parameters. The increase of fish abundance corresponded to increased levels of eutrophication expressed by Secchi-disc visibility and chlorophyll a concentration. A negative correlation between fish and zooplankton abundances was observed, with changes in the zooplankton population structure, caused by fish grazing. Vertical distribution of zooplankton and fish were affected by the thermocline. Maximal concentrations were observed in a well-illuminated surface layer at the highest temperature and oxygen concentration.

Relationship between Secchi disk visibility and chlorophyll a in aquaculture ponds

The potential of using Bannister's linear equation ( k t= k w+ k c c) (where k t is the overall light extinction coefficient, k w represents the non-phytoplankton light extinction, k c is the specific light extinction coefficient due to chlorophyll a (chla), c is the chla concentration, and k c c represents the light extinction due to chla) to partition sources of turbidity in Secchi disk visibility (SDV) measurements in aquaculture ponds was evaluated. Eight data sets from five sites around the world were used in the study. Chlorophyll a data were regressed against the overall light extinction coefficient determined from SDV measurements. The relationship between chla and overall light extinction coefficient was linear for seven of the eight data sets. The contribution of non-phytoplankton turbidity to SDV measurements was estimated by the intercept of the linear regression line (equivalent to k w). The values obtained (range=3.61–8.91 m −1) were variable and unpredictable between replicate ponds at all sites, but did not vary significantly over time ( P<0.05). Because chla concentration serves as an indicator of phytoplankton concentration, the contribution of phytoplankton turbidity to SDV measurements was estimated by the slope of the linear regression line (equivalent to k c) multiplied by the chla concentration. The slope of the regression line (0.014±0.006 m −1 (mg m −3) −1) was similar to values reported for natural freshwater systems. The partitioned light extinction coefficients and chla concentrations were also used to determine the threshold chla concentration above which SDV measurements are determined primarily by chla. The threshold chla concentrations (177–980 mg m −3) above which phytoplankton biomass becomes the primary determinant of SDV were higher than observed chla concentrations. The results indicate that Bannister's linear equation can generally be used to partition and quantify the sources of turbidity in aquaculture ponds. The results also suggest that the contribution of non-phytoplankton turbidity to SDV measurements in fertilized and fed aquaculture ponds can be more important than phytoplankton turbidity.

Salinity, Inorganic Nutrients, and Primary Production in a Shallow Coastal Inlet in the Southern Baltic Sea (Nordrügenschen Bodden) Results from Long‐Term Observations (1960–1989)

AbstractThe Nordrügenschen Bodden waters are a shallow coastal inlet of the Southern Baltic Sea consisting of individual basins, some of which are joined only via small connections. The Bodden are characterized by pronounced horizontal gradients in hydrographical, chemical and biological parameters. Whereas concentrations of inorganic nutrients increase, salinity decreases with increasing distance from the connection to the Baltic Sea. Between 1960 and 1989 long‐term variations of salinity, inorganic nutrients (ammonium, nitrate, phosphate), Secchi disk visibility, and planktonic primary production were monitored at 6 locations throughout the Bodden. The gradients were most pronounced during the first period of observation (1960–1963). Between 1960 and 1985 concentrations of inorganic nutrients in the adjacent Arkona Sea increased up to values several times higher than the previous concentrations. This increased nutrient load in the Baltic Sea was reflected by higher nutrient concentrations and higher primary production in the outer and central parts of the Nordrügenschen Bodden. Between 1963 and 1965 the shipping channel between the outer (Libben) and the inner part (Großer Jasmunder Bodden) of the Bodden chain was dredged. This led to an improvement of water exchange between the Baltic Sea and the Bodden, resulting in a significant reduction of the gradients of chemical and biological parameters. Whereas salinity and Secchi disk visibility increased, inorganic nutrients, and primary production decreased. This was most obvious in the inner parts of the Bodden where the water quality was improved. The trophic status of the Kleiner Jasmunder Bodden is still worrying. This Bodden represents the innermost part of the Nordrügenschen Bodden connected with the Großer Jasmunder Bodden only via a small flood gate. Unless the water exchange is drastically improved, the Kleiner Jasmunder Bodden will retain its hypertrophic status in the longer term. The future development of the trophic status of the Bodden is discussed.

Effects of Increased Nitrogen Fertilization on Water Quality and Yield of Nile Tilapia, Oreochromis niloticus, and Common Carp, Cyprinus carpio, in Earthen Ponds in Egypt

This study investigated the effects of increased nitrogen fertilization on water quality and yield of Nile tilapia, Oreochromis niloticus, and common carp, Cyprinus carpio, in earthen ponds in Egypt. Nine 0.1-ha ponds were filled with water from the Nile River and fertilized once every two weeks with urea and triple super phosphate. Three fertilization regimens were evaluated: (1) no fertilizer (control); (2) low nitrogen treatment in which ponds received 30.9 kg urea/ha plus 30.9 kg triple super phosphate/ha (daily equivalent to 1.0 kgN/ha and 0.4 kg P/ha); and (3) high nitrogen treatment in which ponds received 92.7 kg urea/ha plus 30.9 kg triple super phosphate/ha (daily equivalent to 3.1 kg N/ha and 0.4 kg P/ha). There was a significant increase in total ammonia-nitrogen, nitrate-nitrogen, pH, and chlorophyll awith increased nitrogen in fertilizers; but alkalinity, hardness, total phosphorous, secchi disk visibility, and dissolved oxygen (at 0600 and 1400) were similar. Temperature and secchi disk visibility were the only water quality variables that changed significantly (P<0.05) with season; temperature increased during the summer, but secchi disk visibility decreased. There was no significant difference (P> 0.05) in final body weight, net yield, and reproduction of Nile tilapia among the three treatments. Final body weight and yield of common carp were also similar, regardless of fertilization treatment. It appears that nitrogen did not limit primary production. Low density of tilapia, enhanced turbidity by common carp that affected algal growth and reproduction, and phosphorous limitation may have combined to lower yield in this study.

Survival and growth of juvenile fluted giant clams, Tridacna squamosa, in large-scale grow-out trials in the Solomon Islands

Two large-scale experiments were conducted with juvenile Tridacna squamosa at coastal village sites in the Solomon Islands to provide robust estimates of survival and growth during grow-out. Juveniles were reared to ~ 24 mm shell length (10–11 months) in land-based tanks and then transferred to subtidal sites near villages for grow-out for 8 months. During the first experiment, which started in 1993, survival varied greatly among sites, ranging from 7% to 83%. Exposure (‘fetch area’) and geographic location had a significant influence on survival, indicating that adverse weather conditions affected mortality rates. Inserts made of 5-mm galvanized mesh (‘settlement rings’) fitted to the inside perimeter of grow-out cages significantly reduced loss of clams from cages after transfer to villages. Growth varied from 2.3 to 8.6 mm month −1 and was significantly, positively correlated with the number of clams surviving in the cages. During the second experiment, which commenced in 1994, survival was greater and less variable, ranging from 42% to 83%. Average growth also improved, ranging from 4.0 to 7.2 mm month −1. Growth was positively correlated with water flow, and negatively correlated with Secchi disc visibility and the number of predatory ranellid gastropods found in the grow-out cages. A weak negative relationship between growth and survival occurred during the second experiment. Modifications to aquaculture protocols in 1994 were evaluated in a series of concurrent experiments conducted at the village sites. Grooves in the bases of cages, support of clams using pieces of coral, and retention of ‘settlement rings’ for 4 instead of 2 months had no effect on survival and growth of the clams. However, survival of juveniles preconditioned in the ocean was significantly higher than those reared exclusively in tanks. On the basis of these two experiments, the mean time required to grow T. squamosa from 24 to 50 mm shell length, a size suitable for sale to the aquarium market, was only 5 to 7 months. Our results reveal that village-based farming can be economically feasible in the Solomon Islands, provided reliable hatchery facilities are available.

Feeding activity patterns of the Southern brown shrimp Penaeus subtilis under semi-intensive culture in NE Brazil

This paper investigates the feeding rhythms of Penaeus subtilis under a semi-intensive culture system. The effects of diel variations in water quality and artificial food application on the food intake of the shrimp were also examined. Shrimp were sampled during 24 h periods at 10 day intervals, starting at Day 10 over a 60 day period of the growth cycle. After collection, shrimp had their anterior proventriculus dissected, removed and the contents analysed under a binocular microscope. Water quality was evaluated with an automatic water quality logger, set to automatically conduct 10 min interval readings of temperature, salinity, dissolved oxygen, pH, and ammonia for a continuous 24 h cycle. A total of 869 shrimp had their stomach contents analysed, 3369 individual measurements of water quality (including Secchi disk visibility and pond depth) were conducted. Artificial feeding promoted an enhanced consumption of natural food. Recurrent bouts of food intake occurred soon after the spread of pellets in the early morning and late afternoon. Observed shrimp feeding rhythms were irregular and occurred during both day and night. No significant differences were found between diurnal and nocturnal food consumption levels. Water quality (i.e. temperature, pH, salinity and dissolved oxygen) significantly affected food intake ( P < 0.05), although a low correlation ( R 2 = 0.267) was found. These findings indicate that further research is needed to examine the factors involved in penaeid shrimp feeding behaviour and feeding periodicity under semi-intensive pond systems in order to develop more efficient feed management practices.

Effects of Timing and Duration of Aeration on Water Quality and Production of Channel Catfish

Sccond-year (660 g) channel catfish, Ictalurus punctatus, were stocked in ponds at a density of 1,931 kg/ha and fed a floating, commercial diet at 74 kg/ha/day for 10 weeks. The effect of five aeration treatments on fish growth, survival, and pond water quality was evaluated. The aeration treatments were: no aeration (0 hours), aeration from 0400-0700 (3 hours), aeration from 0400-0600 and again from 1600-1800 (4 hours), aeration from 0100-0700 (6 hours), and continual aeration (24 hours). Dissolved oxygen, temperature, turbidity, Secchi disk visibility, alkalinity, hardness, pH, total and un-ionized ammonia, nitrite, chlorophyll a, and chemical oxygen demand were measured at 5 intervals during the three-month study. Average final weight of fish was significantly affected by aeration. Relative to initial body weight, fish growth in unaerated ponds was lower (- 54%) than that in ponds aerated for 3, 4, or 6 hours (+32, +32, and +23%) or in continually-aerated ponds (+97%) (P < 0.05). Survival was lowest (6...

Production of Sunfish Lepomis spp. in Ponds Treated with Controlled‐Release Fertilizers

Abstract.– Controlled‐release fertilizer in pond water released nutrients according to the pattern reported for exposure in soil. A single application of controlled‐release fertilizer (13% N, 13% P2O5, 13% K2O) was compared with multiple applications of conventional, liquid fertilizer (10% N, 34% P2O5, 0% K2O) for augmenting sunfish production in ponds. Liquid fertilizer was used at an annual phosphorus input rate of 39.3 kg/ha; controlled‐release fertilizer was applied at 19.7, 9.8, and 4.9 kg phosphorus/ha per yr. Phytoplankton abundance inferred from chlorophyll u and Secchi disk visibility estimates and phytoplankton photosynthesis measured by the oxygen, light‐dark bottle method were similar in ponds treated with liquid fertilizer and in ponds that received controlled‐release fertilizers at the two highest rates. Net sunfish production did not differ among ponds treated with 39.3 kg phosphorus/ha in liquid fertilizer or 19.7 and 9.8 kg phosphorus/ha applied in controlled‐release fertilizer. Controlled‐release fertilizer used in this study contained more nitrogen and potassium than needed, but a custom, controlled‐release fertilizer with the proper ratio of nutrients can be made. This fertilizer would have the advantage of requiring only a single application at the beginning of the growing season, and amounts of soluble reactive phosphorus in outflow from ponds could be reduced by using controlled‐release fertilizers.

Effects of Sampling Technique on Precision Estimates for Water Quality Variables in Fish Culture Ponds

Three sampling experiments were conducted to assess variation in water quality variables in ponds: samples from individual ponds on single dates; samples from individual ponds on different dates during a growing season; and samples from replicate ponds. Data on dissolved oxygen, temperature, Secchi disk visibility, total hardness, total particulate matter, filtrable orthophosphate, and pH exhibited wide variation and covered ranges generally expected for fish ponds. A two-stage nested (hierarchical) linear statistical model was used to estimate components of variance for the three experiments. Pond size (0.04 to 10.32 ha) and management scheme (fertilized ponds and ponds receiving daily applications of fish feed) had no influence on variation. In general, viability of water quality components increased in the order: samples within ponds, among sampling dates, and between replicate ponds. Variation was greatest for total particulate matter and filtrable orthophosphate, intermediate for dissolved oxygen, Secchi disk visibility, and total hardness, and lowest for pH and temperature. The components of variance were used to estimate expected standard errors of pond means, seasonal means, and replicate pond means for all water quality variables. Figures prepared from these estimates may be used to determine the numbers of samples, sampling dates, or replicate ponds necessary to achieve a given level of precision.

Water visibility record robust after 3 centuries

Gieskes et al.1 encountered very clear water on October 13, 1986, in a coastal polynya in the eastern Weddell Sea; they recorded a Secchi disc visibility of 79 m at 14.45 UT at 71°23.63'S, 15°02.52'W. They claimed to have “shattered” the “world record” of Berman et al.2 of 53 m in the Mediterranean Sea. They postulated that Secchi disk values in distilled water would be close to 80 m, suggesting that their observation would be the ultimate record. We wish to comment on the record and on maximum Secchi disk values that are possible.

Effects of Pond Volume Manipulation on Production of Fingerling Largemouth Bass,Micropterus salmoides

Hatchery ponds were manipulated by timing of filling to increase the efficiency of production of fingerling largemouth bass, Micropterus salmoides. The effects on pond productivity of fertilizing with chicken manure versus plant meals (consisting of equal parts by weight of cottonseed meal, alfalfa meal, and wheat shorts) were also examined. When ponds were filled in stages, more favorable water temperatures for largemouth bass were achieved, and desirable dissolved oxygen concentrations were maintained. Chlorophyll a levels and plankton community respiration rates were higher in ponds that were fertilized with the plant meals than in ponds fertilized with chicken manure (25% dry matter). Secchi disk visibility was relatively greater in chicken manure ponds than in ponds fertilized with the plant meals; however all differences were not significant. Zooplankton densities were comparable among treatments. Filling ponds in stages had no significant effects on chlorophyll a, plankton community respiration rat...

Tilapia yield improvement through maintenance of minimal oxygen concentrations in experimental grow-out ponds in Honduras

Tilapia yields and water quality were compared in 1000-m 2 grow-out ponds that were unaerated, or aerated beginning at 10 or 30% of oxygen saturation. Tilapia yield and individual final size were significantly greater in aerated ponds than in unaerated ponds, but there were no significant differences between levels of aeration. Treatment means of organic-N, total P. chlorophyll a, net primary productivity, and total volatile solids were not significantly different. However, aeration caused higher clay turbidity as indicated by significantly higher total fixed solids and lower Secchi disk visibility in aerated treatments. Maintenance of oxygen above minimal levels augmented tilapia yields, but additional research is needed to make aerator use in tilapia culture more efficient and profitable.

Spatial and temporal patterns of the zooplankton in the Westerschelde estuary

The invertebrate zooplankton fauna of the Westerschelde (Belgium and The Netherlands) was investigated during 2 yr by means of monthly samples along a salinity gradient. Copepods were usually the most abundant holoplanktonic metazoans except in the freshwater zone where Rotifera were most numerous. The combination of a classification technique and an ordination-regression technique proved to be a valuable tool for the analysis of such an extensive data set. The presence of 4 groups was established, representing spatially distinct populations but with temporally shifting boundaries. Few zooplankton species were truly estuarine in their distribution, but many were derived from nearby coastal waters. This intrusion of marine species started in spring, reaching their most upstream distribution and highest densities in summer-early fall, then declining and retreating from the estuary, disappearing in winter. Fringing this community was a transition group with low densities, but many species. This brackish-water community consisted predominantly of the calanoid copepod Euryternora affinis. It appeared in late fall, spread out seaward to obtain its maximum density and distribution in winter-early spring. Densities then declined and the community was absent by late summer-early fall. The freshwater zone near the port of Antwerp, Belgium, was characterized by a paucity of large zooplankters, despite the high primary production in this zone. This is probably due to the low oxygen availability in this area. A canonical correspondence analysis revealed 2 major environmental axes. The salinity gradient (mainly spatial) explained most of the variance. Strongly correlated with this factor were dissolved oxygen content and secchi disc visibility. The temperature gradient (mainly temporal) was almost perpendicular to the salinity axis, indicating little or no correlation. Of lesser importance was the load of suspended matter, which was highest in the brackish area in autumn-winter. Chlorophyll content of the water was unimportant in explaining community structure. Copepod dry weight was maximal in spring in the brackish part (500 mg m-3); a lower maximum (260 mg mW3) was observed in summer in the marine part of the estuary.

Introgression of the Florida Largemouth Bass Genome into Native Populations in Alabama Public Lakes

Abstract Many public fishing lakes constructed in Alabama first contained the state's native largemouth bass Micropterus salmoides, which has allozyme frequencies similar to those of the pure northern subspecies, M. s. salmoides. Many of these lakes then were supplementally stocked with largemouth bass fingerlings with allozyme frequencies similar to those of the Florida subspecies M. s. floridanus. The supplemental introductions had varying success, and 10 of 11 populations we examined had undergone significant (P < 0.05) allele-frequency changes–representing introgression of Florida alleles-at the diagnostic loci sAAT-B*, sIDHP*, and sSOD*. We evaluated several variables for their effects on introgression: number of years (6–18) since initial stockings of Florida largemouth bass, number of stockings (3–11), stocking densities (334–1,430/hectare), Secchi disk visibility (18–33 cm), and latitude (31°30'–34°90'N). All variables except latitude affected or were correlated with percent introgression, which r...