Cladocerans respond to differences in trophic state in deeper nutrient poor lakes from Southern Norway

We investigated the role of trophic state in structuring the cladoceran assemblages in 35 relatively deep lakes of low nutrient levels in Southern Norway. The lakes cover gradients of altitude, latitude, longitude, area and total phosphorus concentration. The environmental control of the cladoceran assemblages was analyzed using canonical correspondence analysis. Total phosphorus was the most important variable in the minimal adequate model. Hence, the cladoceran assemblage changed with trophic state. Typical clear water species were displaced by indicators of eutrofication with increasing trophic levels. Littoral species constituted the majority of the species recorded. Littoral and pelagic species richness showed a unimodal relationship with trophic state. Along with the change in trophic state and shift in cladoceran assemblage, we observed an increase in the ratio of pelagic to littoral species as well as in the ratio of pelagic efficient bacterial feeders to total pelagic filter feeders. The study indicates that zooplankton could provide a valuable indicator of ecosystem’s structure and function in deeper nutrient poor lakes if included in water quality assessments according to the EU Water Framework Directive. Including sampling of the littoral zooplankton would improve diversity estimates and incorporate the response of the littoral zone to eutrophication.     

The impact of zooplankton size structure on phosphorus cycling in field enclosures

Plastic bag field enclosures (1570 1) were used to investigate the relationship between zooplankton size distributions and the turnover rate of pelagic phosphorus. Water within the enclosures was filtered to simulate either low (200 µm mesh net) or high (80 µm) planktivory. The bags were then spiked with radiolabelled phosphorus and sampled 7 times during the following 24 h period. Zooplankton communities dominated by smaller size classes cycled more phosphorus than those composed of larger species. These experiments reveal that higher trophic level interactions, such as size selective predation, may have a significant impact upon nutrient regeneration rates and, hence, primary production.     

近15年长江口海域海洋生物变化趋势及健康状况评价

对近15年（2004-2018年）长江口海域海洋生物变化趋势进行分析，评价海洋生物健康状况，并对陆源污染物排放与海洋生物变化进行相关性分析。结果表明，（1）长江口海域海洋生物群落结构组成发生了一定变化，与20世纪90年代末相比，浮游植物种类数有所减少，浮游动物、底栖生物种类数有所增加。浮游植物以硅藻为主，但甲藻占比有所增加，2010年以来硅藻、甲藻群落结构进入新的平衡状态；浮游动物以节肢动物为主，主要类群桡足类占比有所下降；底栖生物种类数明显升高。（2）生物多样性总体水平一般，浮游植物多样性指数总体较低，第一优势种的优势度较高；浮游动物多样性指数和丰富度指数多年呈现下降趋势；底栖生物多样性水平一般，优势种渐趋单一。（3）海洋生物总体处于"不健康"状态，主要影响指标为浮游植物密度偏高，浮游动物密度偏低、生物量偏高，底栖动物密度偏高、生物量偏低。生态系统变化与陆源主要污染物排放、营养结构变化及水体富营养化均具有一定的相关性，其中无机氮（DIN）、石油类入海通量与生物健康指数呈显著负相关关系（P < 0.05），无机磷（DIP）与底栖生物生物量呈显著负相关关系。N/P与浮游植物丰度呈显著负相关，但与浮游植物均匀度和多样性指数呈显著正相关；Si/N与浮游植物多样性指数呈显著负相关。海域严重富营养化面积比例与硅藻甲藻种类数比值呈显著正相关关系。     

Influence on phytoplankton biomass in lakes of different trophy by phosphorus in lake water and its regeneration by zooplankton

In 49 unpolluted lakes of north-eastern Poland the biomass of algae in summer is significantly related to the concentration of total phosphorus and to the rate of phosphorus regeneration by zooplankton. Using a model with equations describing these relationships, the biomass of blue-green algae and other phytoplankton groups was predicted for 14 polluted lakes. A good approximation of actual values was obtained only for the biomass of blue-green algae calculated from the estimated rate of P regeneration by zooplankton in these lakes. It is hypothesized that more-or-less edible algae of other classes did not show dependence on the rate of input of regenerated P because their biomass was heavily reduced by grazing of zooplankton.     

Potential impact of size-selective planktivory on phosphorus release by zooplankton

A laboratory model of phosphorus release by Daphnia rosea was implemented for published zooplankton data from lakes characterized by different abundances of size-selective planktivores. Size-selective feeding reduces average prey size and increases P release per unit biomass. At the system level, decreased prey standing crop associated with higher planktivore abundance could balance the size dependent increase in P release rate. However, estimates of both net reduction and net increase in rate of P release from zooplankton resulted from model application. Size-selective feeding might be important not only in energetic or evolutionary relationships between predator and prey but also in determination of the relative importance of different pathways of phosphorus flow through pelagic systems.     

东海赤潮高发区春季浮游动物生态特征的研究

根据2002年4～5月东海122°～123°30′E、29°～32°N水域海洋综合调查资料,对东海赤潮高发区浮游动物的数量分布、群落特征、种类组成及优势种等进行了分析．结果表明,在调查区共鉴定出饵料浮游动物128种[不含16种浮游幼虫(体)和仔鱼],分5门12大类,以桡足类占优势(40种,31．25％)．浮游动物群落以广温广盐生态类群为主．中华哲水蚤(Calanus sinicus)为最主要优势种(142．10ind·m-3,68．09％)．总生物量均值为243．80mg-m3(55．53～773．92mg·m-3),最高密集区(>500mg·m-3)位于长江口30°45′～31°15′N、122°45′～123°15′E水域．饵料生物量均值为195．96mg·m13(55．53～496．09mg·m。),呈长江口(30°30′N以北)水域高于舟山岛东南水域、长江口外海(122°45′E以东)高于近海水域的分布趋势．多样性指数(H′)均值为2．12(1．09～3．73)．长江口水域多样度、均匀度和丰富度低,优势度大,反映出浮游动物群落结构不够稳定．采用逐步回归分析得知,影响本区春季浮游动物生态特征值分布的主要因子是水温、硅藻和甲藻．     

用浮游动物评价巢湖东湖区的水质和营养类型

于2005年6月至2006年6月对巢湖东湖区的浮游动物进行了采样调查,初步研究了巢湖东湖区的浮游动物的种类组成、群落结构特征以及浮游动物物种的多样性,并对巢湖东湖区的水质状况和营养类型作出了评价。研究结果显示浮游动物153种,其中原生动物43属59种,占总种数的38.56%;轮虫类20属48种,占总种数的31.37%;枝角类14属26种,占总种数的16.99%;桡足类20种,占总种数的13.07%。浮游动物的物种多样性随着季节的变化而变化;污生指数法评价结果表明巢湖东湖区的水质状况从2005年6月至2006年6月均处于β-中污带级;用浮游动物数量(ind·L^-1)作为湖泊水体营养程度的生物量指标,其结果显示巢湖东湖区水体从2005年6-9月和2006年3-6月巢湖东湖区水体的营养状况处于富营养状态。     

Effect of vertebrate predation on the spatio-temporal distribution of cladocerans in a temperature eutrophic lake

We analysed the spatio-temporal distribution of zooplankton along a profile of 10 stations from the shore to the pelagic zone from April to September 1988, the period when the larvae and juveniles Rutilus rutilus, the most abundant species in the Lake, are in the littoral zone. The digestive tracts of the young roach were analysed. They fed essentially on rotifers and on cladocerans. For comparison, zooplankton was also analysed at one littoral area without fish fry. There was an increase of cladoceran density from the vegetated nearshore zone to the offshore zone. Considering the density of Bosmina longirostris, Daphnia longispina, Chydorus sphaericus and Ceriodaphnia quadrangula, we observed a different distribution pattern in the course of the year. In the nearshore zone, the relative abundance of small species, Bosmina and Chydorus, was much higher than that of the larger Daphnia. From April to September, predation pressure mainly affected the smallest species: in contrast to the inshore station without fish fry, the density of Bosmina decreased in May in the littoral with fish. Chydorus was concentrated in the littoral between February and April, then grew into the pelagic zone, where predation pressure obviously was low during the warm season. The number of Daphnia, which was eaten by the fish fry at any time, remained low in the nearshore zone, which suggests that the presence of fish may cause Daphnia to avoid this zone. Ceriodaphnia which was not affected by this predation, was scarce in the nearshore zone during mid-summer. The low density of the cladocerans in the nearshore zone is likely associated with vertebrate predation by roach fry and juveniles, the result of such a process being either a depletion in density of the prey, or an avoidance behaviour.     

Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: implications for crustacean zooplankton in littoral and pelagic habitats

The dynamics of crustacean zooplankton in the littoral and pelagic zones of four forest lakes having variable water qualities (colour range 130–340 mg Pt l⁻¹, Secchi depth 70–160 cm) were studied. The biomass of zooplankton was higher in the littoral zone than in the pelagic zone only in the lake having the highest transparency. In the three other lakes, biomass was significantly higher in the pelagic zone than in the littoral zone. In the two lakes with highest transparency, the littoral biomass of cladocerans significantly followed the development of macrophyte vegetation, and cladoceran biomass reached the maximum value at the time of highest macrophyte coverage. In lakes with lowest transparency, littoral zooplankton biomass developed independently of macrophyte density and decreased when macrophyte beds were densest. The seasonal development of the littoral copepod biomass did not follow the development of macrophytes in any of the lakes. The mean size of cladocerans in the pelagic zone decreased with increasing Secchi depth of the lake, whereas in the littoral zone no such phenomenon was detected. Seasonally, when water transparency increased temporarily in two of the lakes, the mean size of cladocerans in the pelagic zone decreased steeply. For copepods, no relationship between water transparency and body size was observed. The results suggested that in humic lakes the importance of the littoral zone as a refuge decreases with decreasing transparency of the water and that low water transparency protects cladocerans from fish predation. All the observed between-lake differences could not be explained by fish predation, but were probably attributed to the presence of chaoborid larvae with variable densities. Feeding efficiency of chaoborids is not affected by visibility and thus they can obscure the relationship between water quality, fish density, and the structure of crustacean zooplankton assemblages. Handling editor: S. I. Dodson     

Seasonal dynamics and grazing rate of zooplankton in Yueqing Bay,China

The species composition,biomass,abundance,and species diversity of zooplankton were determined for samples collected from August 2002 to May 2003 from 14 stations in Yueqing Bay,China.Phytoplankton growth rate and microzooplankton grazing rate were obtained by using the dilution method developed by Landry and Hassett.The spatial and temporal variations of zooplankton and its relationship with environmental factors were also analyzed.The results showed that the zooplankton in the Yueqing Bay could be divided into four ecotypes,namely coastal low saline species,estuary brackish water species,offshore warm water species,and eurytopic species.A total of 75 species of zooplankton belonging to 56 genera and 17 groups of pelagic larva were identified in the Yueqing Bay.The coastal low saline species was the dominant ecotype in the study area,and the dominant species were Labidocera euchaeta,Acartia pacifica,Acrocalanus gibber,Pseudeuphausia sinica,and Sagitta bedoti among others.There was considerable seasonal variation in zooplankton biomass and abundance in the surveyed areas.The peak biomass appeared in August,descending in November and in May,and the lowest biomass appeared in February.Similarly,the highest abundance of zooplankton was observed in August,with the abundance descending in the following months:May,November,and February.There were similar horizontal distribution patterns for the biomass and the abundance of zooplankton.They both increased from the upper to the lower bay in February and May,but decreased from the upper to the lower bay in August.Biomass and abundance were evenly distributed in the Yueqing Bay in November.Moreover,there was marked seasonal variation in the species diversity of zooplankton,which conformed to the abundance of zooplankton.Results of the dilution experiments indicated that there was grazing pressure of microzooplankton on phytoplankton in the Yueqing Bay throughout the year though the rate of microzooplankton grazing on phytoplankton varied seasonally.Phytoplanktons were growing at 0.26-2.07/d and grazed by microzooplankton at a rate of 0.15--0.48/d in different seasons.     

枯水季节赣江近岸水洼与浅滩中浮游动物的群落差异

为探究水洼与邻近浅滩中浮游动物群落是否存在差异, 于2015年1月22日和3月1日, 选择赣江南昌段的3个水洼和邻近浅滩采集浮游动物, 对浮游动物的物种组成、密度、生物量、多样性指数和群落结构进行比较研究。通过单因子方差分析(One-way ANOVA)检验水洼与浅滩中水环境因子及浮游动物密度和生物量的差异。使用非度量多维尺度分析(Non-metric multi-dimensional scaling, NMDS)和群落相似性分析(Analysis of similarity, ANOSIM)对暂时性水洼及邻近浅滩的浮游动物群落进行排序。1月和3月共发现浮游动物44种, 轮虫有30种, 占总捕获数的68%。枝角类和桡足类各有7种, 分别占总捕获数的16%。相对重要性指数(Index of relative importance, IRI)大于1000的优势物种有5种, 分别为汤匙华哲水蚤Sinocalanus dorrii、长肢多肢轮虫Polyarthra dolichoptera、桡足类无节幼体Copepod nauplii、角突臂尾轮虫Brachionus angularis和长圆疣毛轮虫Synchaeta oblonga。单因子方差分析表明, 1月份水洼的水深、电导率和总氮含量显著高于浅滩(P<0.05), 浅滩的水温、pH、溶解氧、浊度和总磷含量略高于水洼。3月份水洼的水深、pH和总氮含量略高于浅滩, 浅滩的溶解氧、浊度和总磷含量较水洼高, 水温和电导率在2种水体中非常接近。1月份水洼中浮游动物的密度(7.90 ind./L)和生物量(7.78 μg/L)均低于浅滩(9.34 ind./L, 19.03 μg/L), 3月份水洼中浮游动物的密度(24.27 ind./L)和生物量(332.14 μg/L)均高于浅滩(22.86 ind./L, 146.10 μg/L)。浅滩中浮游动物的多样性指数(H′)、均匀度指数(J)和物种丰富度指数(D)均大于水洼。非度量多维尺度分析和群落相似性分析表明, 仅1月份水洼与浅滩的浮游动物群落结构具有边缘显著差异(P=0.067), 可大致划分为2个: 水洼群落和浅滩群落。3月份由于水位上涨, 研究的水洼与赣江河道处于连通或半连通状态, 2种生境中浮游动物群落结构无显著差异(P=0.313)。斯皮尔曼相关分析(Spearman correlation coefficient)发现水深和溶解氧是导致水洼和浅滩中浮游动物密度差异的主要环境因子。2种生境中浮游动物的密度与pH、电导率、总氮和总磷均具有显著相关性(P<0.05)。     

Rapid changes in fish community structure and habitat distribution following the precipitation of lake phosphorus with aluminium

1. Fish community structure and habitat distribution of the abundant species roach, perch and ruffe were studied in Lake Nordborg (Denmark) before (August 2006) and after (August 2007) aluminium treatment to reduce internal phosphorus loading. 2. Rapid changes in fish community structure, abundance and habitat distribution occurred following a decline in in‐lake phosphorus concentrations from 280 to 37 μg P L^?1 and an increase in Secchi depth transparency from 1.1 to 1.9 m (August). The proportion of perch in overnight gill net catches increased, whilst roach decreased, and the average weight of all key species increased. 3. The habitat distribution of perch and roach changed from a high proportion in the upper pelagic and littoral zones in 2006, towards enhanced proportions in the deeper pelagic and profundal zone in 2007. The abundance of large‐bodied zooplankton increased and the abundance of benthic invertebrates decreased in the same period, suggesting that the habitat shift was not induced by food limitation. 4. Ruffe shifted from the littoral and upper profundal zones towards the deep profundal zone, likely reflecting an increased predation risk in the littoral zone and better oxygen conditions in the deep profundal. 5. Our results indicate that enhanced risk of predation in the upper pelagic and the littoral zones and perhaps improved oxygen concentrations in the deeper profundal zone at decreasing turbidity are responsible for the observed habitat shift. The results indicate that fish respond rapidly to changes in nutrient state, both in terms of community structure and habitat use.     

Inter- and intra-annual variability in the phytoplankton community of a high mountain lake: the influence of external (atmospheric) and internal (recycled) sources of phosphorus

1. The inter‐ and intra‐annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice‐melting and atmospheric deposition (external). 2. Inter‐annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P‐recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P‐recycling. 3. Year‐to‐year variations in seston C : P and N : P ratios exceeded intra‐annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice‐out (C : P < 100 and N : P < 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios. 4. Intra‐annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice‐out, when the zooplankton community was dominated by copepod nauplii, and high towards mid‐ and late‐season, when these were replaced by copepodites and adults. 5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice‐melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (> 1000%), zooplankton recycling (9–542%), and ice‐melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition. 6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice‐melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load. 7. The elemental composition of the zooplankton explains changes in phytoplankton taxonomic and elemental composition. The elemental negative balance (seston N : P < zooplankton N : P, low N : P recycled) during the thaw, would promote a community dominated by species with a high demand for P (Cryptophyceae). The shift to an elemental positive balance (seston N : P > zooplankton N : P, high N : P recycled) in mid‐season would skew the N : P ratio of the recycled nutrients, favouring dominance by chrysophytes. The return to negative balance, as a consequence of the ontogenetic increase in zooplankton N : P ratio and the external P inputs towards the end of the ice‐free season, could alleviate the limitation of P and account for the appearance of other phytoplankton classes (Chlorophyceae or Dinophyceae).     

Are the banks a source of recolonization after disturbance: an experiment on aquatic vegetation in a former channel of the Rhône River

Hydrolysis of natural dissolved organic phosphorus (DOP) in three hardwater lakes of different trophic level was calculated from kinetic data of phosphatase activity (PA) in different size fractions. DOP as well as kinetics of PA were determined every fortnight in depth profiles during the year 1990. 60% of DOP was assumed to be suitable substrate for phosphatases. The rate of hydrolysis increased markedly with higher trophic level. Average hydrolysis rate of DOP in polytrophic lake Thaler See was 3.26 nM P min^–1 (6 µg P-PO₄ l^–1 h^–1). In oligotrophic Lake Herrensee, dissolved phosphatases were responsible for more than half of the total hydrolysis. In the other two lakes, bacterial and algal surface PA dominated hydrolysis in changing parts depending on kinetics and DOP concentration. The regeneration rate of phosphate by PA was compared to phosphorus (P) excretion rate of zooplankton. Excretion was calculated from zooplankton data and excretion equations from the literature. In oligotrophic Lake Herrensee, excretion by zooplankton recycled in average 18% of the phosphate amount which was hydrolysed from DOP by PA. With higher trophic level, relevance of P excretion from zooplankton decreased drastically.     

Zooplankton of macrophyte overgrowths in the mouth of the Rybinsk reservoir tributary

In the light of the concept of ecotones formed at the boundary of adjacent ecosystems, the species diversity and quantitative characteristics of zooplankton in the macrophyte overgrowths were studied for the first time in the downstream part of the small Il’d’ River, in the open part of the reservoir, and in the zone where their waters contact. The highest development of zooplankton occurs in the littoral zone overgrown with higher aquatic vegetation in the backwaters of the river, which testifies to the presence of the edge effect typical of the ecotone.     

一座热带浅水水库敞水区浮游甲壳动物群落结构

为了解热带中小型浅水型水库枯水期浮游动物群落组成与结构稳定性,于2006至2007的三个枯水期,对位于我国雷州半岛甘村水库敞水区进行了每周一次的高频采样,主要以个体大小作为变量分析了浮游甲壳类动物的群落结构的组成.三次枯水期采样中,敞水区中只检测到枝角类4种和桡足类4种.浮游甲壳类的体长范围多集中在400～700μm之间.枝角类的优势种为模糊秀体溞平均体长为533μm,桡足类成体优势种台湾温剑水蚤的平均体长为787μm,无节幼体和剑水蚤桡足幼体丰度与生物量也相对较高.水库浮游甲壳类种类数和丰度的峰值均出现在体长为300～400μm和400-500μm两个等级内.浮游甲壳类的种类和数量均是以小个体为主,除了营养水平还与水体中的捕食压力紧密相关,鱼类的捕食对于浮游甲壳类群落结构的形成作用显著.     

The stoichiometry of N and P in the pelagic zone of Castle Lake, California

We measured the concentrations, as well as lake-wide amounts,of nitrogen (N) and phosphorus (P) in dissolved, seston andzooplankton pools throughout the water column of Castle Lake,California, during summer, 1991. This allowed us to determinethe stoichiometric ratios of important elements in each pool(C:N, C:P, N:P) as well as for the entire lake. Dissolved andseston pools were the predominant storage compartments for bothN and P; zooplankton never contained >5% of N or 10% of Plake wide. However, by late summer, the concentrations of Pin seston and in zooplankton were similar in the upper portionsof the water column, suggesting that changes in food web structurethat alter zooplankton biomass and community composition (andhence elemental storage in the zooplankton) may produce significantshifts in nutrient storage among pelagic pools. Lake-wide levelsof dissolved N were largely constant over the study period;however, lake-wide dissolved P increased. These dynamics suggestedthat the majority of nutrients stored in dissolved pools wereunavailable for phytoplankton growth. N:P and C:P ratios indicatedthat Castle Lake phytoplankton became severely deficient inP during the course of our observations. These ratios also greatlyexceeded recently reported threshold values for elemental constraintson growth and reproduction for several species of zooplankton.The ratio of N to P in the zooplankton pool was relatively constantand consistently lower than that in the sestion. As a result,the predicted N:P ratio of zooplankton-regenerated nutrientsexceeded the N:P ratio of the seston, implying that zooplanktonnutrient regeneration further skewed N and P supply ratios,and potentially enhanced P limitation of phytoplankton in CastleLake. ¹Present address: Department of Biology, Box 19498, Universityof Texas at Arlington, Arlington, TX 76019, USA     

北部湾近岸海域浮游动物群落结构特征及季节变化

2017年3月(枯水期)、7月(丰水期)和10月(平水期)分别对北部湾近岸海域44个站位的浮游动物进行了调查。结果共检出浮游动物251种和浮游幼体24类,其中枯水期138种(类),丰水期134种(类),平水期191种(类),分属河口低盐、近岸暖温、近岸暖水和外海暖水4个生态类群。优势种9种,其中枯水期以原生动物占绝对优势,丰水期以枝角类、桡足类和浮游幼体类占优势,平水期以十足类和浮游幼体类占优势。浮游动物丰度年均值为789.95个/m~3,呈现出枯水期(1540.19个/m~3)明显高于平水期(457.58个/m~3)和丰水期(372.08个/m~3)的季节变化特征;浮游动物生物量年均值为252.40 mg/m~3,生物量的季节变化与丰度变化不一致,平水期生物量(385.01 mg/m~3)明显高于枯水期(221.41 mg/m~3)和丰水期(150.78 mg/m~3)。多样性指数平水期最高(3.16),丰水期(2.35)次之,枯水期(2.22)最低。枯水期和丰水期北部湾近岸海域浮游动物生物量和丰度水平分布特征基本呈现自河口近岸海域向外海递增的趋势,平水期浮游动物生物量与丰度的空间分布较为均匀。浮游动物的种类组成结构以及优势种的演替对浮游动物的生物量和丰度季节变化有着重要的决定作用。径流导致的悬浮物、营养盐等的变化可能是决定北部湾近岸海域浮游动物生物量和丰度空间分布的主要因素。研究还表明与其他海湾相比,北部湾近岸海域浮游动物群落结构趋于小型化,需加大关注。     

Interactive effects of productivity and predation on zooplankton diversity

Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are less susceptible to predation and if productivity and/or predation pressure are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity or predation and reducing diversity of prey at high levels of productivity or predation. While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator‐prey systems are rare. We tested these predictions using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on diversity of seven zooplankton, cladoceran species that were common in surrounding ponds. Two productivity levels were imposed via phosphorus enrichment at levels comparable to low and intermediate levels found within neighboring natural ponds. We used open systems to allow for natural dispersal and behaviorally‐mediated numerical responses by the flight‐capable predator. Effects of predators on zooplankton diversity depended on productivity level. At low and high productivity, prey species richness declined while at high productivity it showed a unimodal relationship with increasing the predator density. Effects of treatments were weaker when using Pielou's evenness index or the inverse Simpson index as measures of prey diversity. Our findings are generally consistent with model predictions in which predators can facilitate prey coexistence and diversity at intermediate levels of productivity and predation intensity. Our work also shows that the functional form of the relationship between prey diversity and predation intensity can be complex and highly dependent on environmental context.