Seasonal Variation In Density Research Articles

Diversity of Plankton and Seasonal Variation of Density in the Yamuna River in Auraiya District, Uttar Pradesh

Plankton plays a vital role in the ecological dynamics and nutrient cycling of aquatic ecosystems. This research paper aims to investigate the diversity of plankton and their seasonal variation in density in the Yamuna River, located in the Auraiya District of Uttar Pradesh, India. Water samples were collected monthly from multiple sampling stations along the river course for one year. The collected samples were then examined to identify and quantify plankton species using standard protocols. The results revealed significant seasonal variations in the density and composition of planktonic organisms. This study contributes to our understanding of the ecological dynamics of the Yamuna River and emphasizes the importance of monitoring plankton populations for effective river management and conservation strategies.

Earthworm community structure and population dynamics at Minkong forest of Mokokchung, Nagaland, India

Aim: The present study was undertaken to assess the seasonal variations of density, biomass and community characteristics of earthworms in two different types of forest at Minkong area, Mokokchung. Methodology: The survey was conducted during the period from January 2019-February 2020 in two areas viz., mixed forest and plantation. Standard methods were followed to determine the ecological categories of earthworms, density and biomass, relative abundance and species richness as well as for determination of different soil parameters viz., pH, moisture, bulk density, organic carbon, total and available nitrogen, available phosphorus and potassium. Results: 12 number earthworm species viz., Amynthas corticis (Kinberg), Amynthus gracilis (Kinberg), Drawida assamensis (Gates), Drawida hodgarti (Stephenson), Drawida nepalensis (Michaelsen), Drawida sp., Eutyphoeus assamensis (Stephenson), Eutyphoeus festivus (Gates) Metaphire houlleti (Perrier), Perionyx excavatus (Perrier), Perionyx simlaensis (Michaelsen) and Pontoscolex corethurus (Müller) were recorded. Dominant earthworm species in both the areas were D. nepalensis, P. excavatus, Drawida sp. and E. festivus. The Shannon diversity, Simpson diversity and Pielou’s species evenness were 2.17, 0.87 and 0.87 in mixed forest and 2.05, 0.86, 0.86 in plantation. The earthworm density and biomass were greater in plantation (276.74 ind m-2 and 338.47g m-2) compared to mixed forest (230.52 ind m-2 and 254.55 g m-2). Interpretation: As earthworm community characteristics differ depending on the type of forest, therefore, earthworm’ sdiversity and its indices can be used as an ecological indicator of anthropogenic disturbances and can help designing conservation programs for reclaiming the depleted areas. Key words: Biomass, Earthworm diversity, Minkong, Relative abundance, Subtropical ecosystem

Phytoplankton community and driving environmental factors in Lake Dongping after 3 years implementation of the east route of South-to-North Water Diversion Project

为掌握东平湖浮游植物在南水北调东线工程运行后的群落结构变化特征及探究其环境驱动因子，于通水运行3周年后的2017年分4个季节对东平湖的18个样点进行采样调查，同步开展水环境调查.结果显示：共检出浮游植物191种属（包括变种），其中硅藻门、绿藻门、裸藻门、蓝藻门、金藻门、黄藻门、甲藻门和隐藻门分别为64、57、28、22、8、5、4和3种属；优势种属包括蓝藻门的漂浮泽丝藻（Limnothrix planctonica）和色球藻（Chroococcus sp.），绿藻门的小球藻（Chlorella vulgaris）以及硅藻门的小环藻（Cyclotella sp.）、舟形藻（Nauicula sp.）和针杆藻（Synedra sp.）等（优势度Y>0.02）；全年密度和生物量分别为6.74×10⁶ cells/L和5.71 mg/L，并呈现显著季节与空间差异.浮游植物密度季节变化为夏季 > 秋季 > 冬季 > 春季，生物量季节变化为夏季 > 冬季> > 秋季 > 春季；春、夏、秋、冬季物种数分别为71、78、53和125种，Shannon-Wiener多样性指数（H'）均值分别为1.90、1.84、1.76和2.52，Margalef丰富度指数（D）均值为3.07、3.98、2.79和4.75；冗余分析与Pearson相关性分析结果显示，水体综合营养状态指数、高锰酸盐指数、氨氮和水温是影响东平湖浮游植物群落结构的关键驱动因子.对比南水北调东线工程实施之前及通水初期的结果，东平湖浮游植物优势种群无明显变化，密度则较通水初期稍有升高.本研究分析了东平湖浮游植物群落的时空分布特征，发现调水工程可能通过物质带入影响水质，进而引起浮游植物量增加，湖区养殖的退出及增殖放流的实施降低了富营养化风险，但仍存在浮游植物生物量特别是蓝藻生物量升高的风险，建议在调水管理中加强水域生态监测及生境调控技术研究，保障调水水质安全.;In order to investigate the community characteristics of phytoplankton and the determining factors in Lake Dongping after implementation of the east route of South-to-North Water Diversion Project, seasonal samples of phytoplankton and aquatic environment from 18 sampling sites were collected in 2017. A total of 191 taxa of phytoplankton (genera or species) were identified with 64 taxa of Bacillariophyta, 57 taxa of Chlorophyta, 28 taxa of Euglenophyta, 22 taxa of Cyanophyta, 8 taxa of Chrysophyta, 5 taxa of Xanthophyta, 4 taxa of Pyrrophyta and 3 taxa of Cryptophyta. The dominant taxa were Limnothrix planctonica and Chroococcus sp. of Cyanophyta, Chlorella vulgaris of Chlorophyta and Cyclotella sp., Nauicula sp. and Synedra sp. of Bacillariophyta, etc (Y>0.02). The phytoplankton density and biomass were 6.74×10⁶ cells/L and 5.71 mg/L with the seasonal variation of density being summer > autumn > winter > spring and that of biomass being summer > winter > autumn > spring, respectively. About the phytoplankton diversity in four quarters, the number of species was 71, 78, 53 and 125, while the averages of the Shannon-Wiener index were 1.90, 1.84, 1.76 and 2.52 and Margalef diversity index were 3.07, 3.98, 2.79 and 4.75, respectively. Meanwhile, the results of redundancy analysis and Pearson analysis for the relationship between phytoplankton and aquatic environment showed that the factors such as comprehensive nutrition state index, permanganate index, ammonia nitrogen concentration, and water temperature played a key role in affecting the structure and dynamics of the phytoplankton community in Lake Dongping. Compared with before implementation of the east route of South-to-North Water Diversion Project and the initial stage of it, although the density of phytoplankton showed a rise, there was no obvious response in their dominant genera. This study preliminarily verified the spatiotemporal distribution characteristics of the phytoplankton community in Lake Dongping. Total phytoplankton density was found to have increased with the change of water quality caused by the east route of South-to-North Water Diversion Project. The withdrawal of aquaculture and implementation of fishery enhancement and releasing had reduced the risk of eutrophication, but the proliferation of phytoplankton probably increased the risk of algal-bloom in Lake Dongping. Given the changes of phytoplankton community and water quality after water diversion, it is necessary to strengthen the ecological monitoring and controlling in the management of water quality safety and stability.

Return of the Salish Sea Harbor Porpoise, Phocoena phocoena: Knowledge Gaps, Current Research, and What We Need to Do to Protect Their Future

The harbor porpoise (Phocoena phocoena) is one of the most abundant coastal cetacean species in the Northern Hemisphere with differential levels of regional knowledge. Gaps are particularly evident for the Pacific subspeciesPhocoena phocoena vomerina. In the Salish Sea (a transboundary body of water spanning between Washington, United States and British Columbia (BC), Canada), there is a dearth of information on many aspects of the biology, ecology, behavior, sociality, and regionally specific threats. Here we present a case study of the Salish Sea harbor porpoise, combining historical and current research, from both BC and Washington, to provide a more holistic view of this species’ status, the knowledge continuum and gaps, risks from identified threats and what current research and collaborations are revealing about this enigmatic species. The Salish Sea harbor porpoise was abundant to the 1940s and 1950s, but by the 1990s their numbers were greatly reduced, and all but absent in some areas. By the early 2000s, numbers had resurged, and harbor porpoise are now once again found throughout much of the Salish Sea. Despite this, studies focused on Salish Sea harbor porpoises have been limited until recently. Current long-term research has been conducted from vessels and land in both Canada and the United States. Multi-faceted work using techniques including photo-identification (photo-ID), behavioral visual observations, acoustics, commercial fishery surveys, sighting reports, citizen science and other ecological data have provided insight into the seasonal variation in density and abundance, site fidelity, reproduction, by-catch rates, foraging and the identification of important habitats that are used intra- and inter-annually in this region. These may represent culturally and biologically significant habitats for Salish Sea harbor porpoise. Collaborations within and outside of the Salish Sea have revealed consistencies and dissimilarities between different communities or populations; indicating that some aspects are more uniform for the species, while others may be community or population specific. The importance of long-term broad and fine-scale research is highlighted, as well as recommendations to further close the knowledge gaps and reduce the known human threats within the Salish Sea.

Densities of Large Marine Gastropods in Seagrass, Oyster Reef, and Sandy Habitats in Tampa Bay and along the Gulf Coast of Florida

Marine gastropods in Florida are harvested by the marine-life trade, tourists (for shells), and traditional commercial fisheries, but stocks are not formally assessed for most species. The main goal of this project was to estimate biological variables that may be used to evaluate the Species of Greatest Conservation Need status of the banded tulip Cinctura lilium and six other large marine gastropod species that are commonly encountered in the targeted habitats. The principal target, C. lilium, was listed in the 2012 State Wildlife Action Plan. The densities and size structures of C. lilium and six co-occurring species of large marine gastropods in Tampa Bay were examined to evaluate population dynamics, seasonal variation, and habitat preferences of these species. Long-term trends in the densities of large, predatory marine gastropods in 10 regions from 2009 to 2017 were included. In Tampa Bay, strong habitat preferences were observed for all species studied: C. lilium and Melongena corona were most likely to be observed on oyster reefs, and Fasciolaria tulipa was found only in seagrass, whereas Strombus alatus, Triplofusus gigantea, Sinistrofulgur sinistrum, and Fulguropsis spirata were more common on soft sediments. Seasonal variations in densities were weak; seasonal differences in size structure were more pronounced. The smallest individuals of C. lilium appeared in spring and summer; F. spirata, S. alatus, and S. sinistrum reached a detectable size in fall. The smallest detectable M. corona appeared in winter. Regionally, there were more gastropods in southwestern bay grass beds (Tampa Bay, Sarasota Bay, and Pine Island Sound) than in coastal seagrass beds (Big Bend, Citrus, Hernando, Pasco, and coastal Pinellas) or northern bays (St. Joseph Bay and St. Andrew Bay), and each region had characteristic species. As a group, tulip snails exhibited modest local declines in densities at two sites, whereas the whelks and T. gigantea varied annually but did not exhibit consistent trends during 2009 to 2017. The present study establishes a baseline of abundance for seven species of common marine gastropods and recommends expanded monitoring in multiple habitats to improve future analyses of abundance trends.

Biomonitoring of the mesoplanktonic community in a polluted tropical bay as a basis for coastal management

The aim of this study was to describe the seasonal variation in density, diversity, and richness of the mesoplanktonic community within the Guanabara Bay estuary, and to evaluate the relationship between community structure and seasonal variation in environmental and water quality variables. The samples was collected at six fixed stations at the entrance of the bay. The variation between dry and rainy periods influenced the formation of the mesoplanktonic community. The dominance of the exotic species throughout the study period shows a restructuring in the mesoplankton community due to the diverse changes that occur in this environment. The current study shows that the poor sanitary and environmental conditions in Guanabara Bay are aggravated during rainy seasons, and pollution in the area affects the biodiversity of mesoplankton organisms. A research strategy needs be formulated to provide the necessary protocols for the implementation of appropriate management strategies to improve the sanitary conditions and develop a plan of action involving the conservation and restoration of the ecosystem in order to maintain local biodiversity. We suggest continuous monitoring of physical, chemical, and microbiological variables while analyzing the density and diversity of the zooplankton community which responds to changes in water quality.

Seasonal variations of thermospheric mass density at dawn/dusk from GOCE observations

Abstract. Thermospheric mass densities from the GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite for Sun-synchronous orbits between 83.5∘ S and 83.5∘ N, normalized to 270 km during 2009–2013, have been used to develop an empirical mass density model at dawn/dusk local solar time (LST) sectors based on the empirical orthogonal function (EOF) method. The main results of this study are that (1) the dawn densities peak in the polar regions, but the dusk densities maximize in the equatorial regions; (2) the relative seasonal variations to the annual mean have similar patterns across all latitudes regardless of solar activity conditions; (3) the seasonal density variations show obvious hemispheric asymmetry, with large amplitudes in the Southern Hemisphere; (4) both amplitude and phase of the seasonal variations have strong latitudinal and solar activity dependences, with high amplitude for the annual variation at higher latitudes and semiannual variation at lower latitudes; (5) the annual asymmetry and effect of the Sun–Earth distance vary with latitude and solar activity. Keywords. Atmospheric composition and structure (pressure, density, and temperature)

Diversity of Zooplanktons and Their Seasonal Variation of Density in Gulabsagar Water Body, Jodhpur (Rajasthan) During 2014-2016

Diversity of Zooplanktons and Their Seasonal Variation of Density in Gulabsagar Water Body, Jodhpur (Rajasthan) During 2014-2016

Luminescence and density banding patterns in massive Porites corals around the Thai-Malay Peninsula, Southeast Asia

We characterized the annual luminescent and skeletal density banding patterns in 51 massive Porites corals from 15 reefs from six locations around the Thai-Malay Peninsula in Southeast Asia, and explored the seasonal environmental cues/drivers of band formation. Location-specific recurrent annual luminescent banding patterns were found at all study locations with a brighter band occurring toward the end of the year (∼October/November/December) (at five locations) and in ∼June (one location). Annual density banding patterns could only be discerned at four locations, and were categorized into those that formed a dense band commencing ∼November/December, and those starting ∼May/June. Overall, compared to luminescence, variations in skeletal density provided a less clear signal for demarcation of annual growth increments. Seasonal variations in luminescence showed clearest relationships with salinity, as a proxy for freshwater/river runoff. No convincing relationship between intra-annual luminescence intensity and density variations was found, which supports the notion that luminescent banding is due to inclusions of fluorophores into the coral skeleton rather than variations in skeletal architecture. The relationships between seasonal density variations and significant wave height and rainfall suggest density banding in this region is likely related to wave energy, or some other correlated environmental parameter/s. The large variability in skeletal banding patterns not only highlights the current relatively poor understanding of their nature and causes, but also the need for replication in their interpretation, especially in settings with complex seasonal hydrodynamic/hydrological patterns such as those found around the Thai-Malay Peninsula.

Environmental Factors and Seasonal Variation in Density of Mussel Larvae (<i>Mytilus edulis</i>) in Danish Waters

Mussel larval densities may fluctuate considerably on both small spatial and short temporal scales. So far, only few and scattered data on the occurrence of mussel larvae have been reported from Danish waters. However, seasonal variation in density of blue mussel (Mytilus edulis) larvae as related to environmental factors (temperature, salinity, phytoplankton biomass) is basic information of relevance for future line-mussel farming in Danish waters. Here we report on the density of mussel larvae in a number of potential farming sites in the inner Danish waters. The mussel larval density measured in Skive Fjord, an eutrophicated inner branch of Limfjorden, during a period of 20 years, from 1989 to 2009, along with corresponding temperatures and chlorophylla, makes up the most important series of data reported here. In most years, a pronounced spring density peak and a subsequently lower autumn peak could be seen in Skive Fjord, but most conspicuous in the period 1993 to 2002 where the mean maximum spring larval density was 319 ± 260 ind·l-1. Further, data on mussel larval densities have been recorded on 4 locations in the Great Belt region: Kerteminde Bay in 2008 to 2011, and in 2008 at 3 other locations: Musholm Bay, Svendborg Sund, and Horsens Fjord. The maximum spring densities in the studied waters were observed in Skive Fjord, typically in May, whereas 10 to 100 times lower peak densities were found at the other locations studied. The reported observations show that mussel larvae are omnipresent in the studied areas and it is suggested that the larval density is sufficient forrecruitment to future line-mussel farms.

Seasonal variations in the South Atlantic Meridional Overturning Circulation from observations and numerical models

AbstractMonthly climatologies of temperature and salinity from observations and numerical models are used to estimate the Atlantic Meridional Overturning Circulation (AMOC) at 34°S. Observational estimates suggest that the geostrophic transport plays an equal role to the Ekman transport in the AMOC seasonal variations at this latitude, whereas in the models, the Ekman transport controls the AMOC seasonality. The seasonality of the geostrophic transport from observations is largely controlled by the seasonal density variations at the western boundary, but in the models, the eastern boundary dominates. The observed density seasonality at the western boundary is linked to the intensity of the Malvinas Current, which is poorly reproduced in the models. The results indicate that the weak seasonal cycle in the model geostrophic transport can primarily be attributed to excessively strong baroclinicity below the surface mixed layer, whereas the observations show a strong vertical coherence in the velocity down to 1200 m.

Wildebeest (<i>Connochaetes taurinus</i>) Population Densities and Distribution in Dry and Wet Season in the Kilimanjaro Landscape

The conservation of migratory wildlife species in the savannah habitat can be a challenge because of frequent and prolonged drought and their requirements for a large area. We investigated the performance of the wildebeest (Connochaetes taurinus) population in the 25,624 km2 Kilimanjaro landscape of Kenya and Tanzania, which comprises Amboseli-West Kilimanjaro-Magadi-Natron after 2009 drought. We used total aerial counts to determine the spatial distribution and numbers of wildebeests during wet and dry season in 2010 and 2013. Global Positioning System and digital voice recorders were used to count wildebeests along established transects within blocks. There was an increase in the wildebeest population by 103% during the wet season and 14% during the dry season between 2010 and 2013. The seasonal variation in density occurred between the four counting blocks with Natron and Magadi areas recording the highest densities. Generally, the increase in population could be attributed to the recovery of the population after the 2009 drought. The current cross border collaboration between Kenya and Tanzania in aerial surveys is an important step in the conservation of this migratory species in the landscape. This study demonstrates that detailed knowledge of density and spatial distribution of migratory species is required to plan effective conservation action.

Temporal variation of zooplankton on a tropical Amazonian beach

Pinheiro, S.C.C., Magalhães, A., Costa, V.B. da, Pereira, L.C.C. and Costa, R.M. da, 2013. Temporal variation of zooplankton on a tropical Amazonian beach.To assess the spatial and temporal variation in zooplankton populations at Ajuruteua Beach, northern Brazil, monthly samples were taken between August, 2004, and July, 2005, at three fixed stations, during the spring (flood/ebb) tide. The zooplankton community was characterized by the absence of any significant spatial or seasonal variation in density, diversity or evenness. However, density was significantly higher during the ebb tides, with mean values ranging from 4,967±670.0 ind.m−3 in July (flood tide) to 38,213±15,157 ind.m−3 in April (ebb tide). A similar tidal pattern was recorded for the density of copepods. The dominant copepod species were Euterpina acutifrons (47.0±30.5 to 18,313±2,431 ind.m−3) and Paracalanus quasimodo (420.0±60.0 to 11,260±7,203 ind.m−3) and, together with Oithona hebes and O. oswaldocruzi, were responsible for the peaks in zooplankton density recorded in April. Monthly zooplankton diversity (1.8±0.2 to 3.0±0.1 bits.ind−1) was relatively high overall, whereas evenness varied from 0.4±0.0 to 0.8±0.0, with both indices presenting significantly lower values in November. The absence of any significant seasonal trend in total zooplankton during the present study may have been related to atypical precipitation patterns during the study period, when surface waters were abnormally warm (~1.5°C). Overall, zooplankton community structure at Ajuruteua Beach was indirectly related to the local rainfall regime, which affected the physical, chemical, and biological parameters of the water, influencing the temporal dynamics of these organisms.

Habitat shifts and spatial distribution of the intertidal crab Neohelice ( Chasmagnathus) granulata Dana

Intertidal zones of estuaries and embayments of the SW Atlantic are dominated by the semiterrestrial burrowing grapsid crab, Neohelice ( Chasmagnathus) granulata, and characterized by extensive mud flats surrounded by salt marshes. In this work we examined spatial patterns of distribution of N. granulata during two years to explain their movement patterns. The results of the population sampling showed segregation by sex and size throughout the intertidal, with seasonal variations in densities and different condition indices for adults and juveniles at the different zones. The comparison of seasonal activity (ambulatory activity outside burrows) between marshes and mudflats shows that short term (e.g. daily) variations in activity were controlled by tides. Crabs were active at high tides but increased their activity on days with higher tidal amplitude. Seasonal activity showed that at both areas, females remain with low activity except for a peak in winter, while males showed the highest activity during summer in the mudflat zone, but not so in the marsh. This pattern can be the response to differences in stress tolerance, suggesting that high temperatures are limiting the performance of adult crabs during summer, especially at the marsh where physical conditions can be more critical. The spatial size segregation can be explained by differential mortality in each zone (estimated with tethered crabs), and by the juvenile movement between these zones (estimated with movement traps). Juvenile mortality is higher at the mudflat, while adult mortality is higher in the marsh. Smaller juveniles moved to the marsh, where the mortality is lower, and the larger juveniles moved towards the mudflat. This mortality is due almost exclusively to cannibalism, so our results suggest that this movement of different size classes between zones is controlled, at least in part, by intraspecific predation.

Seasonal and Spatial Variation in Diversity and Abundance of Tardigrades in Leaf Litter from Louisiana and Florida

Abstract During 2002–2003, we collected samples of leaf litter (10-cm diameter) from two woodland sites in southwestern Louisiana and one in central Florida. In Louisiana, we collected at four times (autumn, winter, spring, and summer) and in Florida in spring and summer. We divided each sample into two layers, an upper leafy layer (1–2 cm in depth) and a lower layer of humus (1–2 cm in depth). We detected six species of tardigrades in samples from Louisiana and five in samples from Florida. Seasonal variation in density of tardigrades was pronounced at each site. Tardigrades generally were more abundant in the upper layer. Density and diversity of tardigrades in the leaf litter at these sites were less than in mountainous areas of Italy and Tennessee, USA.

Seasonal variation in the composition of fish assemblages in small Amazonian forest streams: evidence for predictable changes

Summary1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages.2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra‐firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort.3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught.4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random.5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site‐specific changes.6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short‐duration lateral migrations to these ponds may play an important role in the seasonal fish‐assemblage dynamics in Amazonian headwater streams.7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.

SEASONAL CHANGES IN SIZE STRUCTURE OF SARGASSUM AND TURBINARIA POPULATIONS (PHAEOPHYCEAE) ON TROPICAL REEF FLATS IN THE SOUTHERN RED SEA(1).

Seasonal variation in density, thallus length and biomass, population size structure, and allometric length-biomass relationships was investigated in populations of Sargassum ilicifolium (Turner) C. Agardh, Sargassum subrepandum (Forssk.) C. Agardh, and Turbinaria triquetra (J. Agardh) Kütz. (Phaeophyceae) on shallow reef flats in the southern Red Sea. Thallus length and biomass varied strongly with season, with the highest values occurring in the cooler months. Thallus densities showed no significant temporal variation. Log-total biomass versus log-density relationships were positive throughout the growth season without any decrease in the slope of the relationship. In two populations, biomass-density combinations approached the interspecific biomass-density line, but the massive annual shedding of modules occurred before self-thinning would set in. Allometric length-biomass relationships varied with season in all populations and were associated with seasonal module initiation, growth, and shedding. Evidence of a strong asymmetric competition was found in two high-density populations. These populations showed a predominance of small thalli during peak development, asymmetrical Lorenz curves, increasing Gini coefficients, and increasing thallus length relative to biomass during the main growth phase. In two other less crowded populations, small thalli were absent during peak development, Lorenz curves were symmetrical, and Gini coefficients decreased during the main growth phase. In these populations, size equalization appears to be due to responses at the modular level rather than size-dependent mortality. We conclude that changes in size structure in this highly seasonal environment are determined by module dynamics, modified by asymmetric competition in some populations, with a minor role of recruitment and no regulatory effect of self-thinning.

Distribution patterns of striped mullet Mugil cephalus in mangrove creeks, Zanzibar, Tanzania

Spatial and seasonal variations in density of striped mullet Mugil cephalus were investigated in four mangrove creeks in Zanzibar, Tanzania, during a one-year cycle. Fish were collected monthly in the lower, intermediate and upper reaches of each creek using a beach-seine net. All fish collected were juveniles between 2 and 16 cm standard length. The density of juvenile mullet differed significantly among the creeks, but the spatial patterns within them were consistent with higher densities upstream in three of the creeks. Generally, small mullet (2–10 cm) were more abundant in the upper reaches compared to the lower and intermediate reaches. Seasonal patterns were weak, although mullet densities were high during the period of heavy rains (March–May). Principal component analysis showed that a muddy substrate with microphytobenthos was positively correlated with high mullet densities, although site-specific variables such as shallow water depth and water clarity were also significantly correlated. Our findings suggest that the densities of juvenile striped mullet vary among sites and creeks in response to refuge availability from turbid, shallow water and the accessibility of food from benthic microalgae.

Temporal variation of sandy beach macrofauna at two sites with distinct environmental conditions on Cassino Beach, extreme southern Brazil

Temporal variations of the macrofauna of sandy beaches have been related to variations in the beach morphodynamics and also to the population dynamics of dominant species. The aim of this article is to describe the temporal variation of the intertidal macrofauna at two sites with distinct environmental condition on Cassino Beach, extreme southern Brazil. At each site three transect lines 50 m apart were defined perpendicular to the shore line, from which samples were collected monthly in triplicate at 4 intertidal levels (10 m apart) from June 2004 to May 2005. During winter a generally low density was observed, due to the absence of recruitments and to the mud deposition, which occurred just before sampling (in April 2004), and to low intensity stranding events. Spring witnessed a population explosion of Scolelepis gaucha, a migration of Mesodesma mactroides adults from the subtidal zone, and a strong stranding event. In the summer, recruitment of M. mactroides, Donax hanleyanus and Emerita brasiliensis was observed. Fall was characterized by low densities, except for D. hanleyanus recruitment. The macrofauna at both sites showed a striking seasonal variation in density and diversity, perhaps attributable to the recruitment of numerically dominant species and physical disturbances (stranding and mud deposition).

Home range size and overlap in female root voles: effects of season and density

In small mammals living in highly seasonal environments, observational studies show that female home range size and exclusiveness are smaller in the nonbreeding winter season than in the breeding summer season. This has led to the notion that nonbreeding females are more social and decrease territorial behavior during winter. However, because territoriality decreases with increasing population density, and density normally increases during the breeding season, the effects of density and season on social structure are usually confounded. To find out which of the 2 factors explains space use, we experimentally established 3 high-density and 3 low-density root vole (Microtus oeconomus) populations in late spring and monitored the populations into the nonbreeding winter season. Population sizes were controlled throughout the breeding period to minimize seasonal variation in density. Home range sizes were larger in founder females than in field-born females but did not change with season or density. Area exclusively used by individual females was lower in winter than summer, and founder females decreased exclusiveness as density increased. We argue that this seasonal pattern of space use might be caused by variation in benefits of group living, whereas founder females also responded to density-dependent competition by reducing area exclusively used. Key words: density, Microtus oeconomus, season, space use. [Behav Ecol]