Seasonal Variation In Abundance Research Articles

Seasonal variation in abundance and size structure of phytoplankton in Baie des Chaleurs, southwestern Gulf of St. Lawrence, in relation to physical oceanographic conditions

We investigated changes in the abundance and size structure of phytoplankton and organic seston in relation to temperature, stratification and current patterns at Gascons on the north shore of Baie des Chaleurs, eastern Canada. Phytoplankton biomass showed a general decrease during the study (May to November 1989), except for a brief diatom bloom in late October. During most of the summer, a strong temperature driven stratification was present and <5 µm cells dominated the phytoplankton community. Particles measuring <5 µm also dominated the particulate organic matter (POC and PON) throughout the year. However, only 40% of these particles could be associated with phytoplankton cells. For both particulate matter and phytoplankton, the abundance of the <5 µm size fraction was positively correlated with the Brunt-Vailsala index of stability of the water column. Inorganic nitrogen may have limited the phytoplankton growth, as generally reported for stratified environments. Most of the biomass was probably supported by nitrogen regenerated through microbial organisms. A large bacterioplankton community was suggested by the abundance of small (<5 µm) non-phytoplanktonic particles with a low and relatively uniform C/N ratio. Larger particles were only abundant at the beginning of the study (May–June) and on one date in October. Their C/N ratios indicated they were of varied origins.

Quantitative and qualitative numerical alterations in splenocyte subpopulations of the cotton rat(Sigmodon hispidus)across seasons

Abstract Previous research in our laboratory has documented seasonal alterations in humoral and cell‐mediated immunity in cotton rat (Sigmodon hispidus) populations. Based on these observations, we hypothesized that these seasonal differences in immune function were attributable in part to qualitative and quantitative numerical changes in specific splenocyte subpopulations. Lymphocytes were harvested from spleens of 139 cotton rats collected from a tallgrass prairie in central Oklahoma from December 1991 to September 1992. Unique splenocyte subpopulations were identified using fluorescein conjugated cell surface markers (concanavalin‐A, peanut agglutinin, soybean agglutinin, Helix pomatia agglutinin, pokeweed mitogen, and rabbit‐anti‐rat immunoglobulin‐G). All subpopulations examined were more abundant in fall and winter than spring and summer. Several plausible explanations for seasonal variation in abundance of splenocyte subpopulations are discussed.

Further observations on zooplankton of the Potengi Estuary (Natal, Rio Grande do Norte, Brazil) with special reference to the larvae of Brachyura (Crustacea, Decapoda)

The present paper deals with data on zooplankton collected from the estuary of Potengi, Natal, Rio Grande do Norte, Brazil covering a period of one year from October 1992 to October 1993. Three fixed stations within the estuary located at varying distances from the mouth of the river were sampled. Sampling was done monthly when the tide was lowest. Analysis of the samples has shown clear pattern of seasonal variations in abundance of the dominant zooplankton components which are composed of Copepoda, Appendicularia, larvae of Brachyura, larvae of Cirripedia and Chaetognalha. A detailed study of the larvae of Brachyura was also attempted which showed that seven species can be identified in the samples, of which those of Aratus pisonii (H. Milne Edwards) and Cardisoma guanhumi Latreille have already been described. Other five species are designated here numerically and description of various stages present in the samples are given.

Habitat use by the fishing spiderDolomedes triton in a northern Everglades wetland

We investigated patterns of habitat use by adult fishing spidersDolomedes triton in a seasonally dynamic freshwater wetland of the northern Everglades. Spiders were collected monthly (July 1990-January 1992) in adjacent wet prairie, alligator hole, and sawgrass habitats. Overall density of adult fishing spiders during the study period was 0.1 spiders per m2, although there was extensive seasonal variation in abundance of these semi-aquatic arthropod predators. Densities were significantly higher in sawgrass stands than in wet prairies or alligator holes, which did not differ from one another. Sawgrass sites were characterized by shallower water depths, taller and denser canopy structure, more extensive litter deposits, and fewer potential avian and piscine predators. Overall, these data suggest that structural complexity and habitat heterogeneity influence the demographics of the fishing spider in this Everglades marsh system.

Characteristics of Feeding Guilds and Variation in Diets of Bird Species of Three Adjacent Tropical Sites

We studied feeding guild characteristics of birds in northeastern Venezuela in three adjacent tropical habitats: thorn scrub, thorn woodland, and deciduous forest. We mist-netted and marked birds twice monthly to obtain data on site tenacity. We determined diet by examining regurgitated food samples. Species were assigned to feeding guilds by a cluster analysis based on the proportion of the different food types found in their stomachs. Characteristics of each feeding guild from each habitat were submitted to a principal components analysis. Five parameters clearly distinguished feeding guilds: number of species, proportion of transient individuals, amount of seasonal variation in abundance, mean body mass, and estimated biomass. These parameters also appeared to be related to diet characteristics (food types taken, diet diversity) at the species level. Differences among habitats in food availability led to strong between-site differences in the diet of several species, and, accordingly, in their temporal and spatial patterns of abundance.

Seasonal dynamics of dominant species of arbuscular mycorrhizae in burned and unburned sand prairies

Seasonal variation in abundance of spores of arbuscular mycorrhizal fungi was examined for four growing seasons on burned and unburned sand prairies in Illinois. The three leading mycorrhizal species were an undescribed species of Glomus, Scutettospora heterogama, and Sclerocystis rubiformis, in order of decreasing abundance. Mycorrhizal spore abundance for all species generally showed similar seasonal patterns on burned and unburned sites during the sampling period, i.e., minimal in the middle of the growing season and maximal near the end of the season. Individual species, however, exhibited different seasonal patterns of sporulation or sporulation strategies. Burning caused a 2-year depression in spore abundance for Glomus species and Scutellospora heterogama, and a 3-year long depression in spore abundance for Sclerocystis rubiformis. The seasonal patterns observed were more discernible in root zone than in randomly collected soil. Spore abundances were always significantly lower on burned sites than on unburned sites early in the growing season. The results of this study, and studies conducted parallel to this one, suggest that the greater decline in spore abundance in burned sites than in unburned sites early in the growing season may be indirectly due to greater root growth, and (or) related changes in host physiology, in response to fire. Key words: mycorrhizae, fire, sand prairie, grassland, sporulation, inoculum, fungi.

Onchocerciasis in British cattle: a study of the transmission ofOnchocercasp. in North Wales

The transmission of bovine Onchocerca spp. in Britain was investigated by studying the Ceratopogonidae and Simuliidae which attacked bait cattle between April and October at two sites in North Wales, where either Onchocerca lienalis and O. gutturosa (Cynwyd), or O. lienalis alone (Pentrefoelas) occurred. Based on the relative abundance of each species, their seasonal variation in abundance and preferential feeding sites on cattle, the presence of Onchocerca sp. infective larvae and the development of patent infections in bait cattle, vectors were identified as Simulium ornatum s.1. at Pentrefoelas and S. reptans at Cynwyd (0.5% and 1.5% of parous flies infective, respectively). There was no significant difference between the lengths of the infective larvae from vectors at either site (Pentrefoelas: mean = 543.5 microns, SD = 29.45; Cynwyd: mean = 550.86 microns, SD = 35.26; p > 0.1), which were consistent with descriptions of O. lienalis. The identity of a vector for O. gutturosa was not determined and the role of certain candidates is discussed. In this respect, the absence of Culicoides nubeculosus was notable. Transmission of Onchocerca sp. was limited to a short period of about 8 weeks, by a combination of vector abundance and parasite uptake from dermal skin layers, and a mean daily temperature requirement of 15 degrees C or more for complete parasite development in the vector. The effect of this temperature restriction in relation to the geographical distribution of the two species is discussed.

Seasonal variation in abundance and diet of the sand goby Pomatoschistus minutus (Pallas) in a northern Baltic archipelago

Abstract In shallow, soft-bottom benthic communities meiofauna is as important as macrofauna, with regard to both biomass and production. Shallow bottoms are also important as feeding habitats for the sand goby Pomatoschistus minutus and meiofauna accounts for a considerable part of their diet. Population dynamics, diet and potential predation effect on the infauna by P minutus were studied in a shallow (1—1.5 m) sandy area in the northwestern part of Aland, northern Baltic Sea (May-October, 1989).The predation efficiency of the fish was studied in aquaria with Ostracoda, Corophium volutator; Neomysis integer and Macoma balthica as prey. The abundance of P. minutus resembled that of other areas in the Baltic and neighbouring seas. The lowest density was found in October (0.5 ind/m2), while the highest abundance (11.2 ind/m2) was recorded in September. The diet of P. minutus varied seasonally and with increasing fish size. Small individuals ( 40 mm) a...

No evidence for seasonality in the ectoparasitic fauna of slimy mackerel, Scomber australasicus

Fifteen (or sixteen) species of ectoparasites-6 monogeneans, 4 (or 5) trematodes, 2 copepods, 1 isopod, 1 cestode, and unidentified cysts -were collected from 428 Scomber australasicus individuals caught at Eden, New South Wales, Australia, between November 1988 and June 1992. Neither common nor rare parasites of slimy mackerel showed seasonal variation in abundance or prevalence. Only the copepod Brachiella magna occurred more frequently on larger (older) fish than smaller (younger fish). Two monogeneans (Kuhnia scombri, Pseudokuhnia minor), 2 didymozoids, and unidentified cysts occurred more frequently on smaller than larger fish. The intensities of the remaining monogeneans (Kuhnia scombercolias, Kuhnia sprostonae and Grubea australis), 1 didymozoid, 1 copepod (Peniculus sp.), the isopod (Ceratothoa imbricata) and a trypanorhynch cestode were not significantly correlated with fish length.

Population dynamics and host relationships of neotropical fruit flies (Diptera: Tephritidae) in seasonal and non‐seasonal environments

Abstract Seasonal variation in abundance of the fruit flies Anastrepha striata, A. obliqua and Ceratitis capitata was determined by monitoring adults in food‐baited standard McPhail traps, during 14–28 month periods at seven sites with different habitats and elevations in Costa Rica. Population fluctuations were evaluated with respect to phenology of host trees and rainfall. The peak number of fruit flies captured was significantly correlated with the fruiting peaks of their major hosts. There was no direct relationship between rainfall and variation in number of flies trapped, except for C. capitata which was more abundant in drier habitats and periods of less precipitation. The duration of the adult emergence period declined as temperatures rose. Oviposition and feeding behaviour, host infestation rates, sex ratios, and responses to coloured traps, are also presented and discussed.

Seasonal responses of a fringing reef community to eutrophication (Reunion Island, Western Indian Ocean)

Seasonal changes in benthic community composition were investigated on a degraded fringing reef subject to nutrient-enriched submarine groundwater discharge. From December to February (hot season), algae dominate the benthic commuruty, overgrowing scleractinians: 19 % of hard corals died in the studied area, of which 16 % were overgrown by algae (turfs mostly composed of Ceramiales, Dictyosphaeria setchellii and encrusting coralline algae) and 3 % were bleached and subsequently died. In a qualitative assessment, the rhodophyte Gracilaria crassa appeared to be a major factor in the overgrowth and mortality of corals on the reef flat. Algal components displayed strong seasonal variation in abundance and dominance. Changes in the algal community are highly correlated with 2 environmental factors : (1) early onset of intermittent trade winds in March, correlated with replacement of large fleshy algal facies by lithophytic algal facies and (2) calm conditions in November, associated with warming of the waters, correlated with an increase in large fleshy algae capable of overgrowing many substrata. Thus, during the hot season, hydrological conditions cause eutrophic conditions and enhance algal growth while reducing coral vitality. Therefore, this is the most critical period for a coral community growing in eutrophic conditions.

Phenology of fleshy fruits in a wet sclerophyll forest in southeastern Australia: are birds an important influence?

The hypothesis that seasonality in the production of fleshy fruits in temperate regions is the result of selection by avian seed dispersal agents or avian seed predators was tested in a temperate wet sclerophyll forest in south-eastern Australia. I determined whether ten common fleshy-fruited species produced fruits when potential avian seed dispersers were most abundant or avian seed predators were least abundant. The season of fruit production was not correlated with avian disperser abundance nor with seasonal changes in avian seed predators. Peak fruiting occurred during autumn but fruit-eating birds were equally abundant from spring through to autumn. Avian seed predators (parrots) did not show any seasonal variation in abundance. If dispersers are influencing fruiting seasonality in wet sclerophyll forest, it is likely that changes in social behaviour and feeding patterns of dispersers during autumn, rather than increases in abundance, will be an important influence on fruiting patterns in wet sclerophyll forest. However, environmental and life history factors may also influence phenology.

Population Biology of the Freshwater Snail Biomphalaria pfeifferi in the Zimbabwe Highveld

1. The population dynamics of Biomphalaria pfeifferi (Krauss), an intermediate host for intestinal schistosomiasis, were studied at four river sites in the Zimbabwe highveld for up to 28 months. 2. Adult snail abundances were monitored at weekly intervals using a fractional sampling method. 3. At two sites a mark-recapture method was used to estimate absolute abundances and recruitment and loss rates. 4. Seasonal variations in abundance were related to temperature and water flow. Mortality rate was not significantly influenced by temperature but recruitment rate increased over the range 13-24°C

An analysis of survey measurements of tsetse challenge to trypanotolerant cattle in relation to aspects of analytical models of trypanosomiasis

The recent development of analytical models of trypanosomiasis has increased the general applicability of models to the strategic control of the disease. An analysis of detailed data on tsetse abundance and infection rates and trypanosome prevalence in village-based trypanotolerant cattle over 4 years in The Gambia showed that seasonal patterns of abundance in Glossina morsitans-infested areas were consistent, and that the rates of trypanosome infection remained relatively unchanging. However, there were two distinct seasonal trypanosome prevalence patterns in cattle, with peaks occurring either in May/June/July or November/December. The peaks of trypanosome prevalence therefore occurred either 4 months before or after the times of peak challenge from G. morsitans, not 1 or 2 months after as predicted by analytical models. In G. palpalis-infested areas there was little seasonal variation in abundance or trypanosome infections, but peak trypanosome prevalence still occurred mostly in June/July. Despite the incongruity between the months of peak prevalence and challenge, the overall annual prevalence rates and tsetse challenge indices showed a significant linear relationship. It is concluded that existing analytical models need to be refined to take into account trypanotolerance and the various influences on the expression of this trait.

Temporal Variation in Birds and Fruits Along an Elevational Gradient in Costa Rica

We studied temporal fluctuations in fruit production by plants and in populations of understory fruit—eating birds at three elevations (50, 500, and 1000 m) in Costa Rican wet forests over a 12—16 mo period to investigate effects of resource variation on bird movements and community structure. We used mist nets to monitor changes in frugivore abundance, migration patterns, and breeding and molting cycles. We sampled understory fruits of each forest concurrent with studies of frugivores. Both frugivores and fruit exhibited considerable seasonal variation in abundance. Peak frugivore capture rates occurred during peak periods of ripe fruit abundance. Altitudinal migrants left lower montane (1000 m) forest during periods of fruit scarcity and wee present in lowland (50 m) and foothill (500 m) forest when ripe fruit was abundant. Migrants, both altitudinal and temperate, accumulated fat before migration, and perhaps (for altitudinal migrants) in anticipation of breeding. Some residents also put on fat before breeding. Breeding was seasonal at all forests and occurred when ripe fruit abundance was low. Results of this study indicate that birds may track changes in resource abundance. Thus, variation in resource abundance influences dynamics of bird communities, both in terms of species composition and abundance. Further, results illustrate the importance of viewing communities from different scales; dynamics at a local scale (e.g., one elevation) can be influenced by changes in conditions (e.g., fruit abundance) elsewhere. That some species regularly moved along elevational gradients implies that preservation of many species and of the biotic integrity of entire systems may require conservation of large, connected blocks of suitable habitat.

Seasonal variation in abundance and species composition of choanoflagellates (Acanthoecideae) at Antarctic coastal sites

The abundance of loricate choanoflagellates at three shallow inshore sites (< 30 m) near Davis, Antarctica from March 1982 to February 1983 showed marked variation which followed the seasonal cycle of chlorophyll a concentration. In summer the concentration of choanoflagellates ranged from 104–106 cells 1−1 while in winter they were in the order of 102 cells 1−1. Species seasonal succession was observed with Bicosta spinifera, Crinolina aperta and Parvicorbicula socialis, organisms which dominated the summer population, being absent in winter whereas Cosmoeca ventricosa and Saepicula leadbeateri were found only in winter. Acanthocorbis unguiculata, Calliacantha simplex and Diaphanoeca grandis were present throughout the year.

Monsoon influences on micronekton of the Banda Sea (Indonesia)

Spatial and seasonal variations in abundance, biomass, composition, horizontal and vertical distribution of mesopelagic fishes and crustaceans of the eastern Banda Sea sampled by a RMT 8 during the southeast monsoon (August 1984) and northwest monsoon (February 1985) are described. Total micronekton biomass decreased by about one third from August (0.14 g C·m −2) to February (0.10 g C·m −2), less than, but in accordance with, observed differences on other trophic levels in the pelagic ecosystem. The seasonal differences in total biomass were largely due to differences in specific weight of the organisms, rather than to changes in abundance. Spatial variation in micronekton biomass was largest during the southeast monsoon, with relatively low values in the area with most upwelling, the Aru Basin. Biomass distributions differed for micronekton, zooplankton and phytoplankton, indicating that upwelling in these waters occurs spotwise both spatially and in time. During the northwest monsoon the micronekton was more uniformly distributed. The taxonomic composition of mesopelagic fish and crustaceans, studied by sorting in some major groups, did not show large differences in relative abundances. Most groups decreased from August to February, except for stomatopods and leptocephali, which increased. Differences in the adult/juvenile ratios between the seasons varied per group, but in all cases juveniles were present in both periods, suggesting a continuous reproduction with seasonal differences in intensity. The micronekton did not show seasonal changes in the vertical distribution and diel migration.

Seasonal variation in lipid class and fatty acid composition of two small copepods in Balsfjorden, northern Norway

Zooplankton were collected at Svartnes, northern Norway between 1985 and 1986. The lipid class and fatty acid composition of a subarctic population ofPseudocalanus acuspes (Giesbrecht, 1881) andAcartia longiremis Lilljeborg are described for four seasons of the year. The results are discussed in relation to the seasonal variation in abundance and dry weight of the predominant developmental stages. The lipid composition of these small copepod species resembles that of omnivorous or carnivorous species.P. acuspes andA. longiremis overwinter in a state of arrested development, but seem to feed actively throughout the year. This behaviour resembles that of the omnivoreMetridia longa (Lubbock), but is in contrast to that of the herbivoreCalanus finmarchicus (Gunnerus) from the same area, which spends the winter in diapause.

Seasonal variation in abundance of the Indian mackerel, Rastrelliger kanagurta Cuvier (Pisces: Scombridae) along the Zanzibar coast of East Africa

Purse-seine catches of the mackerel, Rastrelliger kanagurta Cuvier along the west coast of Zanzibar revealed seasonal patterns of abundance associated with monsoon winds. Low catches were obtained during the south-east monsoons while the north east monsoons were characterised by high catches. The largest sizes of mackerel were caught between June and August with a mixture of large and smaller ones between September and March. Mackerel in spawning condition were common between June and September with immature ones between September and January. The variations in mackerel abundance, size composition, and maturity of the fish are discussed in relation to seasonal changes, fish migrations, recruitment and food supply.

Pelagic origin and fate of sedimenting particles in the Norwegian Sea

A 17 month record of vertical particle flux of dry weight, carbonate and organic carbon were 25.8, 9.4 and 2.4g.m −2y −1, respectively. Parallel to trap deployments, pelagic system structure was recorded with high vertical and temporal resolution. Within a distinct seasonal cycle of vertical particle flux, zooplankton faecal pellets of various sizes, shapes and contents were collected by the traps in different proportions and quantities throughout the year (range: 0–4,500 10 3m −2d −1). The remains of different groups of organisms showed distinct seasonal variations in abundance. In early summer there was a small maximum in the diatom flux and this was followed by pulses of tinntinids, radiolarians, foraminiferans and pteropods between July and November. Food web interactions in the water column were important in controlling the quality and quantity of sinking materials. For example, changes in the population structure of dominant herbivores, the break-down of regenerating summer populations of microflagellates and protozooplankton and the collapse of a pteropod dominated community, each resulted in marked sedimentation pulses. These data from the Norwegian Sea indicate those mechanisms which either accelerate or counteract loss of material via sedimentation. These involve variations in the structure of the pelagic system and they operatè on long (e.g. annual plankton succession) and short (e.g. the end of new production, sporadic grazing of swarm feeders) time scales. Connecting investigation of the water column with a high resolution in time in parallel with drifting sediment trap deployments and shipboard experiments with the dominant zooplankters is a promising approach for giving a better understanding of both the origin and the fate of material sinking to the sea floor.