Macrobenthic community structure before and after pollution abatement in the Neches River estuary (Texas)

Macrobenthos and physicochemical conditions in the lower 39 km of the Neches River estuary were studied from August, 1984 to May, 1985. The results were compared with data collected in 1971–1972. Between 1972 and 1984 the permitted BOD waste load in the tidal Neches River was reduced from 123 125 kg d to 8717 kg d. River discharge and dissolved oxygen concentrations were consistently higher and salinity was lower, during the same seasons, during the 1984–1985 study. A total of 50 taxa of macrobenthos were collected in 1971–1972 and 104 taxa were collected in 1984–1985. The numbers of taxa per collection at each station in 1984–1985 were at least twice those found in 1971–1972. Minimum densities in 1984–1985 were much higher than the maximum densities in 1971–1972 at all stations. Patterns of dominance, Sorenson's similarity index, and diversity ( ) values indicated improved water quality in 1984–1985. Statistical analysis of macrobenthic diversity indicated significant differences between upper estuary and lower estuary stations in 1971–1972. No significant differences were found in 1984–1985. Significant differences in numbers of taxa, macrobenthos densities, and values between the two studies were found. Reductions of waste loads, increased river discharge, and deepening of the navigation channel were among the factors that probably contributed most to the changes in community structure of the macrobenthos observed.     

Long-term variation of outdoor radon equilibrium equivalent concentration

Long-term variation of outdoor radon equilibrium equivalent concentration was investigated from 1982 to 1992 at a semi-natural location 10 km north of Munich, southern Germany. For this period the continuous measurement yielded a long-term average of 8.6 Bq·m^–3 (arithmetic mean) and 6.9 Bq·m^–3 (geometric mean), from which an average annual effective dose of 0.14 mSv due to outdoor radon can be derived. A long-term trend of the radon concentration was not detectable over the whole period of observation. However, by time series analysis, a long-term cyclic pattern was identified with two maxima (1984–1986, 1989–1991) and two minima (1982–1983, 1987–1988). The seasonal pattern is characterized by an autumn maximum and an early summer minimum. On average, the seasonal maximum in October was found to be higher by a factor of 2 than the June minimum. The diurnal variation of the radon concentration shows a maximum in the early morning and a minimum in the afternoon. On average, this maximum is a factor of 2 higher than the minimum. In the long term a seasonal pattern was observed for diurnal variation, with an average diurnal maximum to minimum ratio of 1.5 in winter compared with 3.5 in the summer months. The radon concentration is correlated with a meteorological parameter (stagnation index) which takes into account horizontal and vertical exchange processes and the wash-out of aerosols in the lower atmosphere.Dedicated to Prof. F. Waschsmann on the occasion of this 90th birthday     

Macrobenthic community structure before, during, and after implementation of the Clean Water Act in the Neches River estuary (Texas)

Community structure of macrobenthos in the industrialized, tidal Neches River was studied 12 years after implementation of phase III of the Clean Water Act (CWA). Result were compared with a 1971–72 study conducted before implementation of the CWA and a 1984–85 study conducted after implementation of phases I & II of the CWA. The permitted BOD waste load decreased 93% between the 1971–72 and 1984–85 studies, then increased 19% between 1984–85 and the current study. A total of 50 taxa were collected during the 1971–72 study, 104 during the 1984–85 study, and 110 during the current study. Numbers of taxa per collection at each station increased by a factor of at least two between the 1971–72 and 1984–85 studies and were significantly different at the 0.0001 level. Numbers of taxa per collection at each station were not significantly different between the 1984–85 and the current study (p=0.286). Minimum collection densities in 1984–85 were higher than maximum densities in 1971–72 and were significantly different at the 0.0001 level. Collection densities were not significantly different between the 1984–85 study and the current study (p=0.374). Shannon's annual and station collection diversity values significantly increased (p=<0.05) between the 1971–72 and 1984–85 studies, but not between the 1984–85 and the current study (p=>0.05). Dendrograms of Sorenson's similarity index between all pairs of stations were more alike between the 1971–72 and the current study than between the 1984–85 and the current study. Evidence of some organic enrichment at the upper stations was indicated by the dominance of Limnodrilus hoffmeisteri and depressed oxygen concentrations. Environmental quality in the river has greatly improved since implementation of the CWA in 1972. Most of the improvments were due to phases I & II of the CWA, with little apparent change since implementation of phase III.     

Demersal fish assemblages associated with hard bottom habitat in the South Atlantic Bight of the U.S.A.

Synopsis Demersal fish communities associated with hard bottom habitats in the South Atlantic Bight were investigated in three depth zones (inner, middle and outer shelf) between 30° and 33°N latitudes. Fishes were sampled with trawls and baited fishing gear, and were observed by remotely operated underwater television. Most demersal hard bottom fishes demonstrated seasonal differences in abundance in each depth zone, especially at the inner and outer shelf stations. Diversity values from trawl catches were higher in winter than summer at inner and outer shelf stations, but lower in winter at middle shelf stations. Species richness was higher in summer than in winter at most stations, but H' diversity patterns were more influenced by community evenness. Diversity values were higher than those reported for similar depths in the Middle Atlantic Bight. Mean biomass of demersal teleosts for all stations combined was slightly greater in winter than in summer. There was no significant difference in biomass between stations in summer, however, middle shelf stations had significantly greater biomass than inner or outer shelf stations in winter. Biomass estimates from the hard bottom areas studied were considerably higher than those reported in the literature for sand bottom areas in the South Atlantic Bight, but less than those reported for tropical reefs. Cluster analysis revealed differences in community composition between day and night trawl tows at all stations, and greater seasonal differences in species composition at inner and outer shelf stations than at middle shelf depths. Underwater television provided useful complementary data to trawl catches, documented the presence of large fishes which avoided the trawl, and provided information on the community composition at high relief stations which could not be trawled.     

Long-term monitoring of the coral reef fish communities around a nuclear power plant

Over the past 21 years (1979–1999) we have observed temporal changes in the fish communities on a coral reef around a nuclear power plant in southern Taiwan. Data used for analyses were collected bimonthly by scuba-diving ichthyologists at four sub-tidal stations (Stations A, B, D, E). The commercial operation of the nuclear power plant was launched in the summer of 1984. During the study period the number of fish species varies, with the coefficient of variation (CV) ranging from 19.0% (Station A) to 25.2% (Station D). Nevertheless, the sequential data on number of species follow a random trend in terms of runs up and down at all four stations. This characteristic persists both before and after the initiation of power plant operation. Dendrograms drawn using UPGMA (unweighted pair-group method using arithmetic averages) on the dissimilarity coefficients between yearly fish occurrences show that the years 1980–1984 are more closely grouped than any other years. This phenomenon prevails at all stations, indicating that wide-scale change occurred between 1984 and 1985. After the power plant began operation, changes in water temperature were minute at these sub-tidal stations. Impacts from other sources such as chlorine release and fish impingement seem remote. We believe temporal variations in the studied fish communities can be better explained as arising from natural fluctuations of environmental factors as well as physical disturbance caused by typhoons. The latter factor is also thought to account for the major faunal change between 1984 and 1985.     

In-situ observations on the distribution and behavior of amphipods and Arctic cod (Boreogadus saida) under the sea ice of the High Arctic Canada Basin

The occurrence and behavior of sympagic amphipods and Arctic cod (Boreogadus saida) were studied in the High Arctic Canada Basin by diving under the ice at seven stations in summer 2002. Still images of video-transects were used to obtain animal abundances and information on the structure of the ice environment. Mean amphipod abundances for the stations varied between 1 and 23 individuals m^–2, with an increase towards the western part of the basin. The standard deviation within the 31–51 images analyzed per station was small (<1 individual m^–2). Gammarus wilkitzkii was found in low abundances, often hiding in small ice gaps. Small amphipods (Onisimus spp., Apherusa glacialis, and juveniles of all species) tended to move freely along the bottom of the floes. B. saida occurred in narrow wedges of seawater along the edges of melting ice floes at three stations in water depths of 10–50 cm and was never found under the ice. The fish occurred in schools of 1–28 per wedge. Fish were inactive and did not escape the approaching diver. Resting in the wedges may be a strategy to reduce energetic requirements and avoid predators.We dedicate this publication to Professor Dr. J. Lenz (Kiel University).     

Amphipod crustaceans as an important component of zoobenthos of the shallow Antarctic sublittoral

Benthic quantitative samples were taken in 1988 in the soft bottom sublittoral of Admiralty Bay (King George Island, South Shetlands) using a Tvärminne-type bottom sampler and SCUBA-diving technique at 7 successive stations situated at depths from 4 to 30 m.Dominant animal groups in terms of abundance were Amphipoda, Polychaeta and Bivalvia, whereas in terms of biomass Echinoidea were also dominant. Amphipod crustaceans clearly dominated the zoobenthos at depths from 10 to 25 m (the numerical share surpaising 60%) with maximal abundance of abt. 17 000 ind m^–2; in terms of biomass at specific depths amphipods occupied the 1st, 2nd or 3rd place with maximal biomass of abt. 100 g m^–2 where the maximal total biomass of zoobenthos reached 260 g m^–2 (10 m).Amphipoda were the most diversified group with some 35 taxa belonging to 14 families. Most species belonged to Eusiridae s.l. and Lysianassidae s.l. Dominant forms were Pontogeneiella brevicornis, Prostebbingia gracilis, Schraderia gracilis, Hippomedon kergueleni, Orchomenella cf. ultima, Cardenio paurodactylus and Paraphoxus rotundifrons.     

Seasonal patterns of nutrient deposition in a Quercus douglasii woodland in central California

The monthly deposition of total nitrogen, phosphorus, potassium, calcium and magnesium via canopy throughfall, and various components of the litterfall was measured for 31 months under mature Quercus douglasii and in the bulk precipitation in the surrounding open grassland. Seasonal patterns of nutrient concentration in leaf litter, throughfall, and precipitation were also measured. Total annual subcanopy deposition exceeded open precipitation deposition by approximately 45–60x for nitrogen, 5–15x for phosphorus, 30–35x for potassium, 25–35x for calcium, and 5–10x for magnesium. Total annual subcanopy deposition was low in comparison to other oak woodland sites reported in the literature. Throughfall and leaf litter were the primary sources of nutrients and thus determined the seasonal peaks of nutrient deposition. The first autumn rains and leaf fall were associated with one peak in nutrient deposition, and throughfall during early spring leaf emergence was associated with a second peak in potassium, magnesium and phosphorus. Non-leaf plant litter (excluding acorns) provided approximately 15–35% of most nutrients, with twigs and bark depositing over 12% of the annual calcium flux in 1987–1988, and flower litter depositing over 8% of the annual nitrogen flux in 1986–1987. Acorns had high concentrations of phosphorus and nitrogen and during the mast season of 1987–1988 they contained a large proportion of the total subcanopy annual flux of these elements. With acorns excluded, total annual nutrient deposition was similar between years, but timing of nutrient deposition differed. Late summer leaf fall associated with drought, variation in precipitation, and variation in deposition of non-leaf parts were associated with seasonal differences in nutrient deposition between years.     

Inorganic phytoplankton nutrients in the Wadden Sea areas off Schleswig-Holstein. II. Dissolved ortho-phosphate and reactive silicate with comments on the zooplankton

In the summer months of 1984, 1986, 1987 and 1988, several stations off the west coast of Schleswig-Holstein were investigated with regard to the situation of ortho-phosphate, reactive silicate and mesozooplankton. An increase in phosphate was observed, whereas an increase in silicate could only be found in the areas influenced by the River Eider. Polychaete larvae show a decreasing tendency in the summer months; this is thought to be influenced by water temperature.     

Effects of tide range alterations on salt marsh sediments in the Eastern Scheldt,S. W. Netherlands

The construction of three dams induced large changes in the tide range in the Eastern Scheldt, a tidal inlet in the southwestern part of the Netherlands. In 1986 the mean high tide level was reduced by almost one metre. This excluded the greater part of the medium-high marshes almost completely from tidal flooding.The absence of tidal flooding in the summer of 1986, combined with a net precipitation deficit, increased the bulk density of the sediment irreversibly from 400 to 530 kg m^–3 in the top 5 cm. The subsidence of the backmarshes varied from 1 to 8 cm.The loss of moisture allowed oxygen to diffuse into the initial reducing sediment. This altered the geochemistry of the sediment significantly. The redoxcline was lowered from a mean depth of about 15 cm in 1985 to 20–30 cm in the summer of 1986. Evidence of pyrite oxidation within a narrow depth interval of 15–30 cm was obtained from the change in the composition of pore waters. The rapid increase in redox potentials (up to 600 mV) and total dissolved iron (up to 5 mM 1^–1) and SO _inf4 ^sup2– (up to 65 mM 1^–1) and the decrease in pH (up to 4.5–2.5) all suggest a rapid oxidation of pyrite. Acidic conditions were found only in sediments with low contents of calcium carbonate and high contents of pyrite. The importance of seasonal changes in redox processes on the partial decalcification of the salt marsh sediments is discussed.The established of a new tide range was reflected in the pH and Eh of the sediment. In December 1987 the depth profiles of pH and Eh were again close to those observed in 1985.     

Retention of nitrogen and phosphorus in a Danish lowland river system: implications for the export from the watershed

In a Danish lowland river system intensive measurements were made, in four 80 m reaches, of the nitrogen (N) and phosphorus (P) stored in the stream sediment. The results were used for calculation of the total retention in the river system during two summers (June to August). In addition, the mobilization of nutrients from the stream bottom in autumn 1987 was compared with the export from the watershed.During the study period (June 1987 to September 1988) the amounts of N and P stored in stream reaches were determined fortnightly using a core-sample technique. In reaches dominated by submersed macrophytes, 25–40 g N m^–2 and 20–30 g P m^–2 were stored during two summers, against only 10–15g N and P m^–2 for sandy and gravely reaches. In riparian zones with emergent macrophytes the retention was even higher than in the submersed macrophytes. Gross retention exceeded net retention by a factor of two to three.Net retention of P in the river system during the summer of 1987 was equal to the summer export from the watershed. On an annual basis, retention in the summer constituted 20% of the P export. In contrast, retention in the summer of 1988 amounted to 60% of the total P export during the same period (38% reduction) and 22% in comparison with the annual export. The corresponding figures for N were lower, showing reductions of 16% and 12% of the export of total N in the two summer periods, and about 1% of the annual exports.In September 1987 6.4% of the total N export and 65% of the total P export from the watershed consisted of resuspended material. In 1987 the N and P retained during the summer was almost completely resuspended during storm events during September to November.     

Macrobenthos in Montedoglio Reservior, central Italy

An investigation was carried out on the macrobenthos of Montedoglio Reservoir, an artificial lake in central Italy. Seven qualitative and quantitative samplings, performed by drag netting and airlifting at 5 stations, revealed a total of 48 taxa during 1992–94. The highest density and most frequent terms were Tubificidae, Chironomidae of the genera Procladius, Tanypus, Chironomus and Polypedilium and Chaoboridae. Tubificidae were found mainly at a depth of 7–9 m, Chironomidae at 1–3 m. The modest density and low biological diversity values are probably attributable to the substrate characteristics and a drop in hypolymnic oxygen during thermic stratification. The oxygen stress at the lake bed, which was unexpected given the scarce planktonic productivity, may have been caused by recent impoundment.     

Living benthic foraminifera of the Okhotsk Sea: Faunal composition, standing stocks and microhabitats

Live (Rose Bengal stained) benthic foraminifera were investigated in surface sediment samples from the Okhotsk Sea to reveal the relationship between faunal characteristics and environmental parameters. Live benthic foraminifera were quantified in the size fraction > 125 µm in the upper 8 cm of replicate sediment cores, recovered with a multicorer at five stations along the Sakhalin margin, and at three stations on the southwestern Kamchatka slope. The stations are from water depths between 625 to 1752 m, located close or within the present Okhotsk Sea oxygen minimum zone, with oxygen levels between 0.3 and 1.5 ml l^- 1. At the high-productivity and ice-free Kamchatka stations, live benthic foraminifera are characterized by maximal standing stocks (about 1700-3700 individuals per 50 cm²), strong dominance of calcareous species (up to 87-91% of total live faunas), and maximal habitat depths (down to 5.2-6.7 cm depth). Vertical distributions of total faunal abundances exhibit a clear subsurface maximum in sediments. At the Sakhalin stations, which are seasonally ice-covered and less productive, live benthic foraminifera show lower standing stocks (about 200-1100 individuals per 50 cm²), lower abundance of calcareous species (10-64% of total live faunas), and shallower habitat depths (down to 2.5-5.4 cm depth). Faunal vertical distributions are characterized by maximum in the uppermost surface sediments. It is suggested that 1) lower and strongly seasonal organic matter flux, caused by the seasonal sea ice cover and seasonal upwelling, 2) lower bottom water oxygenation (0.3-1.1 ml l^- 1), and 3) more pronounced influence of carbonate undersaturated bottom water along the Sakhalin margin are the main factors responsible for the observed faunal differences. According to species downcore distributions and average living depths, common calcareous species were identified as preferentially shallow, intermediate and deep infaunal. Foraminiferal microhabitat occupation correlates with the organic matter flux and the depth of the oxygenated layer in sediments.     

Rotifer community structure in the south basin of Lake Biwa

Seasonal changes in abundance and species composition of rotifers were surveyed at five locations under different physical and chemical conditions in the south basin of Lake Biwa during July 1987–June 1988. Total density of rotifers showed similar seasonal fluctuation with three peaks, although the maximum density showed north (low)-south (high) gradient. Polyarthra spp. (P. vulgaris and P. dolichoptera with low density) dominated except during July–October 1987 and April–May 1988. In the former period the species of Brachionus, Trichocerca, Filinia and Hexarthra, and in the latter, those of Synchaeta, Keratella and Kellicottia had somewhat different proportions in the communities of north and south stations, respectively. The difference in composition during July–October suggests a difference of trophic state between the northern and southern areas in the south basin of Lake Biwa. However, the dominance of Polyarthra and the difference in the composition during April–May 1988 could not be explained by such a difference in trophic state. No critical difference was observed in the community structure of the eastern area of the south basin, where the seasonal fluctuations in nutrient levels and phytoplankton community structure were different from other areas in the south basin. The present results, therefore, suggest that physical and chemical conditions were not effective in controlling the rotifer community structure in the south basin of Lake Biwa.     

History and dynamics of the crown-of-thorns starfish Acanthaster planci (L.) in the Suva area,Fiji

Annual recruitment of Acanthaster planci over a 13 year period in a known juvenile refuge substratum on Suva Reef was very patchy in time and space. Macroscale recruitment events resulted in outbreaks over many thousands of hectares over 100 km of coastline whereas localised but intensive mesoscale events resulted in concenrated aggregations over areas of several tens of hectares. A very low level of recruitment occurred in most years but three intensive, macroscale recruitment events occurred in 1977, 1984 and 1987, and two localised but intensive recruitment events occurred in 1982 and 1983. Annual recruitment of A. planci between 1975–89 had no overt relationship with rainfall events, as proposed in the terrestrial run-off hypothesis on the cause of primary out-breaks. Growth rates within monitored cohorts were highly variable, but mean growth rates of two cohorts 1984 and 1987 were relatively similar. Mortality rates of three monitored cohorts were variable. Longevity of the 1977 cohort was 7–8 years, while that of the 1984 cohort was 2–3 years. Mortality of the 1987 cohort has been low to date (i.e. 4 years of age). Disease, attributed to an undescribed sporozoan pathogen, was responsible for two mass mortalities in the 1984 cohort, in 1984 and in 1986, and eventually resulted in the extinction of this cohort. Anecdotal reports indicate that outbreaks of A. planci are increasing in frequency and severity. According to Fijian reef fishermen, the starfish were low to moderate in abundance in the Suva area from the 1920s to the 1960s. A progressive build up occurred on some reefs in 1963–67 leading to a major outbreak episode from 1967–1971. The outbreaks were in a chronic phase between 1977–1989. Our findings demonstrate that there is no invariable pattern or process involved in the population dynamics of A. planci and that a single explanation of the A. planci phenomenon is therefore unlikely.     

Seasonal Rotifer Dynamics in the Long-term (1969–2002) Record from Lake Kinneret (Israel)

Long-term (1969–2002) data record of biomass distribution of rotifers in Lake Kinneret is combined with previously published information on their metabolic activity and newly calculated population dynamics parameters to synthesize a model of their seasonal dynamics in Lake Kinneret. Nineteen rotifer species were recorded in routine samples collected in Lake Kinneret (Israel) in 7 offshore (deeper than 5 m), stations, at 12 discrete depths during 1969–2002. Organisms were sorted and counted (including external egg carrying females), biomass was measured and calculated for the entire lake stock (g_w.w m⁻²; mg l⁻¹). Rates of grazing, respiration and production were measured experimentally at three different temperature ranges. Results were extrapolated to the lake community for months with similar temperatures. Rotifera comprised 7% of total zooplankton biomass in Lake Kinneret whilst Cladocera and Copepoda 58 and 35% respectively. Rotifers were found to be more abundant during December–June and decline in summer months. Monthly (1969–2001) means indicated total grazing capacity of rotifers as 11%, respiration as 9% and production as 3.7% of the total zooplankton metabolic activity. Positive relations were indicated between rotifer and small bodied cladoceran numerical concentrations. Population growth models suggest that rotifers are not food limited in Lake Kinneret but that fish predation plays an important role in regulating abundance in spring-summer and fall.     

Long-term changes in intertidal flat macrozoobenthos as an indicator of stress by organic pollution

Organic waste-water from beetsugar factories is being discharged by pipeline into the eastern part of the Dutch Wadden Sea since 1969 mainly in September–December. The organic waste load was high in the first four years and diminished considerably from 1974 onwards.From 1968 macrobenthos has been monitored at five permanent stations. Two of these stations were situated relatively close to the outfall of the pipe-line.Data on numerical density of seven macrobenthic species were analysed for synchrony of yearly fluctuations in order to discriminate between pollution-induced changes in population densities and changes due to variation of natural environmental factors.A great deal of concordance in density fluctuations was found between the five stations. Comparison with data from the western part of the Dutch Wadden Sea (150 km apart) revealed a relative good concordance of the density fluctuations in eastern (only 3 stations) and western Wadden Sea. The two stations close to the outfall showed an aberrant pattern of density variations due to the waste discharge.     

Fragilariopsis cylindrus (Grunow) Krieger: The most abundant diatom in water column assemblages of Antarctic marginal ice-edge zones

Summary Planktonic diatoms were sampled in the ice-edge zone of the Bellingshausen Sea during the early austral spring of 1990 and of the Weddell Sea during the late spring of 1983, the autumn of 1986, and the winter of 1988. The four cruises in the Antarctic marginal ice edge zones, combined with the summer cruise in Prydz Bay during a brief ice-free period (1988) provided us with opportunities for spatial and seasonal studies of diatom abundance and distribution in the water column. Cells from discrete water samples from 73 stations near the marginal ice-edge zones during all seasons were counted to gain quantitative information on the composition, abundance, and distribution of diatoms. Diatom abundance was dominated by the pennate diatom, usually nanoplanktonic, Fragilariopsis cylindrus (Grunow) Krieger, during all five cruises. The highest integrated numbers of F. cylindrus were found during the summer cruise with 7.9 × 10¹⁰ cells m^–2 and the lowest numbers were found during the winter cruise with 1.1 × 10⁸ cells m^–2. The average integrated abundance of F. cylindrus from the five cruises was about 35% of the total diatom abundance. The overall spatial pattern of F. cylindrus near the marginal ice-edge zones during the five seasonal cruises were similar with the highest number of cells in open waters compared to ice-covered waters. When all 73 stations during the five cruises were included in the correlation analysis, the abundance of total diatoms was positively correlated with the abundance of F. cylindrus, suggesting that the ice-edge pulses of diatom assemblages in the water column largely reflected its abundance. Cluster analysis revealed that the stations in marginal ice-edge zones were not only separated by seasons and locations, but they also separated based on location of stations in relation to the ice edge (open water stations vs. ice-covered stations).     

Inorganic phytoplankton nutrients in the Wadden Sea areas off Schleswig-Holstein. I. Dissolved inorganic nitrogen

In the summer months of 1984, 1986, 1987 und 1988, several stations off the west coast of Schleswig-Holstein were investigated with regard to the situation of dissolved inorganic nitrogen. An increase in nutrients was observed during the last few years related to a decrease in salinity. This is possibly due to an increased freshwater outflow caused by higher rainfall during recent summers.     

Artemia monica cyst production and recruitment in Mono Lake,California, USA

Annual egg production was determined for Artemia monica in Mono Lake, California, from 1983 to 1987. Annual oviparous (overwintering cyst) production was 3 and 7 million cysts m^–2 yr^–1 in 1986 and 1987, respectively, as measured by in situ sediment traps. Cyst production for the entire five year period was calculated using Artemia census data and inter-brood duration derived from mixolimnetic temperature. These estimates ranged from 2 to 5 million cysts m^–2 yr^–1. This method underestimated annual production by 30%, when compared to estimates using sediment traps. Cyst production was similar during 1983–1986 and showed a significant increase in 1987, which was due primarily to a larger reproductive population later in the year. Recruitment into the adult populations of the following spring ranged between 1.4 to 3.2%. Overall abundance of this generation reflected the patterns in annual cyst production. Compensatory effects must operate on the second generation of each year, since summer populations were similar in all years despite differences in cyst production.