Characteristics of the microzooplankton community in Jiaozhou Bay, Qingdao, China

The species composition and abundance of microzooplankton at 10 marine and five coastal stations (Hongdao, Daguhe, Haibohe, Huangdao and Hangxiao) in the Jiaozhou Bay (Qingdao, China) were studied in 2001. The microzooplankton community was found to be dominated by Tintinnopsis beroidea, Tintinnopsis urnula, Tintinnopsis brevicollis and Cvdonellopsis sp. The average abundance of microzooplankton was highly variable among stations. Specifically, the abundance of microzooplankton was higher at inshore stations and lower in the center of the bay (St. 5), bay mouth (St. 9) and outside the bay (St. 10). The highest average annual densities (346 ind./L) was observed at St. 3, while the lowest (55 ind./L) was at St. 10. Two abundance peaks were recorded in May (324 ind./L) and February (300 ind./L). The distribution of microzooplankton in three sampling layers at the 10 stations was relatively homogenous and the abundance decreased slightly as the water depth increased. At coastal stations, the highest average annual density was recorded at Hongdao Station (677 ind./L), followed by Daguhe Station (616 ind./L), Haibohe Station (400 ind./L), Huangdao Station (275 ind./L) and Hangxiao Station (73 ind./L). Furthermore, a 24-h sampling analysis conducted at Hangxiao Station revealed that the microzooplankton assemblages were characterized by a bimodal diel vertical migration pattern, with the highest densities occurring at dusk (154 ind./L), followed by dawn (146 ind./L), noon (93 ind./L) and midnight (77 ind./L). The density of microzooplankton in the Jiaozhou Bay was in the middle range of the densities of temperate coastal waters worldwide.

Close coupling between phytoplankton growth and microzooplankton grazing in the western South China Sea

We conducted 28 dilution experiments during August-September 2007 to investigate the coupling of growth and microzooplankton grazing rates among ultraphytoplankton populations and the phytoplankton community and their responses to habitat variability (open-ocean oligotrophy, eddy-induced upwelling, and the Mekong River plume) in the western South China Sea. At the community level, standing stocks, growth, and grazing rates were strongly and positively correlated, and were related to the higher abundance of larger phytoplankton cells (diatoms) at stations with elevated chlorophyll concentration. Phytoplankton growth rates were highest (> 2 d(-1)) within an eastward offshore jet at 13 degrees N and at a station influenced by the river plume. Among ultraphytoplankton populations, Prochlorococcus dominated the more oceanic and oligotrophic stations characterized by generally lower biomass and phytoplankton community growth, whereas Synechococcus became more important in mesotrophic areas (eddies, offshore jet, and river plume). The shift to Synechococcus dominance reflected, in part, its higher growth rates (0.87 +/- 0.45 d(-1)) compared to Prochlorococcus (0.65 +/- 0.29 d(-1)) or picophytoeukaryotes (0.54 +/- 0.50 d(-1)). However, close coupling of microbial mortality rates via common predators is seen to play a major role in driving the dominance transition as a replacement of Prochlorococcus, rather than an overprinting of its steady-state standing stock.

Estuarine nutrient loading affects phytoplankton growth and microzooplankton grazing at two contrasting sites in Hong Kong coastal waters

To investigate the effects of enhanced nutrient loading in estuarine waters on phytoplankton growth and microzooplankton grazing, we conducted monthly dilution experiments at 2 stations in Hong Kong coastal waters with contrasting trophic conditions. The western estuarine station (WE) near the Pearl River estuary is strongly influenced by freshwater discharge, while the eastern oceanic station (EO) is mostly affected by the South China Sea. Growth rates of phytoplankton were often limited by nutrients at EO, while nutrient limitation of phytoplankton growth seldom Occurred at WE due to the high level of nutrients delivered by the Pearl River, especially in the summer rainy season. Higher chlorophyll a, microzooplankton biomass, phytoplankton growth and microzooplankton grazing rates were found at WE than at EO. However, the increase in chlorophyll greatly exceeded the increase in phytoplankton growth rate, reflecting different response relationships to nutrient availability. Strong seasonality was observed at both stations, with temperature being an important factor affecting both phytoplankton growth and microzooplankton grazing rates. Picophytoplankton, especially Synechococcus, also exhibited great seasonality at EO, with summer abundances being 2 or 3 orders of magnitude higher than those during winter, Our results confirm that in eutrophic coastal environments, microzooplankton grazing is a dominant loss pathway for phytoplankton, accounting for the utilization of >50%, of primary production on average.

Studies on growth rate and grazing mortality rate by microzooplankton of size-fractionated phytoplankton in spring and summer in the Jiaozhou Bay, China

Dilution experiments were performed to examine the growth rate and grazing mortality rate of size-fractionated phytoplankton at three typical stations, inside and outside the bay, in the spring and summer of 2003 in the Jiaozhou Bay, China. in spring, the phytoplankton community structure was similar among the three stations, and was mainly composed of nanophytoplankton, such as, Skeletonema costatum and Cylindrotheca closterium. The structure became significantly different for the three stations in summer, when the dominant species at Stas A, B and C were Chaetoceros curvisetus, Pseudo-nitzschia delicatissima, C. affinis, C. debilis, Coscinodiscus oculus-iridis and Paralia sulcata respectively. Tintinnopsis beroidea and T. tsingtaoensis were the dominant species in spring, whereas the microzooplankton was apparently dominated by Strombidium sp. in summer. Pico- and nanophytoplankton had a relatively greater growth rate than microzooplankton both in spring and summer. The growth rate and grazing mortality rate were 0.18 similar to 0.44 and 0.12 similar to 1.47 d(-1) for the total phytoplankton and 0.20 similar to 0.55 and 0.21 similar to 0.37 d-1 for nanophytoplankton in spring respectively. In summer, the growth rate and grazing mortality rate were 0.38 similar to 0.71 and 0.27 similar to 0.60 d-1 for the total phytoplankton and 0.11 similar to 1.18 and 0.41 similar to 0.72 d(-1) for nano- and microphytoplankton respectively. The carbon flux consumed by microzooplankton per day was 7.68 similar to 39.81 mg/m(3) in spring and 12.03 similar to 138.22 mg/m(3) in summer respectively. Microzooplankton ingested 17.56%similar to 92.19% of the phytoplankton standing stocks and 31.77%similar to 467.88% of the potential primary productivity in spring; in contrast, they ingested 34.60%similar to 83.04% of the phytoplankton standing stocks and 71.28%similar to 98.80% of the potential primary productivity in summer. Pico- and nanophytoplankton appeared to have relatively greater rates of growth and grazing mortality than microphytoplankton during the experimental period. The grazing rate of microzooplankton in summer was a little bit greater than that in spring because of the relatively higher incubation temperature and different dominant microzooplankton species. Microzooplankton preferred ingesting nanophytoplankton to microphytoplankton in spring, while they preferred ingesting picophytoplankton to nanophytoplankton and microphytoplankton in summer. Compared with the results of dilution experiments performed in various waters worldwide, the results are in the middle range.

Spatial distribution of ciliates, copepod nauplii and eggs, Engraulis japonicus post-larvae and microzooplankton herbivorous activity in the Yellow Sea, China

The abundance of anchovy Engraulis japonicus larvae, >20 mum ciliates, copepod eggs and nauplii, and microzooplankton herbivorous activity were studied in the Yellow Sea in June 2000. Anchovy juveniles and larvae were found in only 6 of the 19 stations sampled. The ciliate communities were dominated by 2 species: Laboea strobila and Strombidium compressum. In the surface waters, the abundance of L. strobila ranged between 0 and 560 ind. l(-1). S. compressum only appeared at Stns 15 to 18 (20 to 3300 ind. l(-1)). L. strobila was found mainly in the top 20 m. The abundance of L. strobila was less than 50 ind, l(-1) in waters deeper than 25 m. S, compressum showed subsurface abundance peaks at the salinity abnormality. Tintinnids occurred occasionally with abundance lower than 100 ind. l(-1), The total ciliate abundance fell in the range of 40 to 3420 ind. l(-1). The ciliate biomass in the surface water and the water column ranged between 0,15 and 6.76 mug C l(-1) and 0.4 and 134.4 mg C m(-2), respectively, In the surface waters, the abundance of copepod eggs and nauplii ranged from 0,3 to 3.1 and 1,1 to 15.6 ind, l(-1), respectively. The average abundance of copepod eggs and nauplii in 4 depth (0, 5, 10 and 20 m) fell in the range of 0.2 to 2.8 and 1.0 to 29.4 ind. l(-1), respectively. As a food item of the E. japonicus post-larvae, the abundance of copepod nauplii and eggs appeared to be low. The abundance peaks of ciliate and E, japonicus post-larvae coincided. Although not found in the gut of E, japonicus post-larvae, aloricate ciliates might be ingested by first-feeding anchovy larvae, preventing initial starvation and prolonging the time to irreversible starvation. On the basis of dilution experiments with positive microzooplankton grazing rates, microzooplankton grazed at rates of 0 to 0.61 d(-1). Grazing pressure of microzooplankton on chlorophyll a standing stock (P-i) and potential chlorophyll a primary production (P-p) were 17 to 46% and 35 to 109% d(-1), respectively.

Rapid changes in stocks of ciliate microzooplankton associated with a hurricane in the Bohai Sea (China)

The ciliate community in the Bohai Sea (China) was studied from 23 September to 7 October 1998. A hurricane struck the study area between the 2 grid station investigations; which were 6 d apart. The ciliate biomass decreased drastically after the hurricane. The surface ciliate biomass decreased from 0.2-12.3 to 0.02-2.8 mug C l(-1) and the water column ciliate biomass from 2-136 to 0.01-47 mg C m(-2). The ciliate biomass:chlorophyll ratios in the surface water were calculated to be 0.04 to 4.7 (1.41 on average) during the first grid investigation and 0.01 to 1.18 (0.26 on average) during the second grid investigation. Distinct patterns of temporal changes in ciliate abundances were found at different stations both before and after the hurricane.

Fast microzooplankton grazing on fast-growing, low-biomass phytoplankton: a case study in spring in Chesapeake Bay, Delaware Inland Bays and Delaware Bay

Dilution experiments were performed to examine the growth and grazing mortality rates of picophytoplankton (< 2 mu m), nanophytoplankton (2-20 mu m), and microphytoplankton (> 20 mu m) at stations in the Chesapeake Bay (CB), the Delaware Inland Bays (DIB) and the Delaware Bay (DB), in early spring 2005. At station CB microphytoplankton, including chain-forming diatoms were dominant, and the microzooplankton assemblage was mainly composed of the tintinnid Tintinnopsis beroidea. At station DIB, the dominant species were microphytoplanktonic dinoflagellates, while the microzooplankton community was mainly composed of copepod nauplii and the oligotrich ciliate Strombidium sp. At station DB, nanophytoplankton were dominant components, and Strombidium and Tintinnopsis beroidea were the co-dominant microzooplankton. The growth rate and grazing mortality rate were 0.13-3.43 and 0.09-1.92 d(-1) for the different size fractionated phytoplankton. The microzooplankton ingested 73, 171, and 49% of standing stocks, and 95, 70, and 48% of potential primary productivity for total phytoplankton at station CB, DIB, and DB respectively. The carbon flux for total phytoplankton consumed by microzooplankton was 1224.11, 100.76, and 85.85 mu g C 1(-1) d(-1) at station CB, DIB, and DB, respectively. According to the grazing mortality rate, carbon consumption rate and carbon flux turn over rates, microzooplankton in study area mostly preferred to graze on picophytoplankton, which was faster growing but was lowest biomass component of the phytoplankton. The faster grazing on Fast-Growing-Low-Biomass (FGLB) phenomenon in coastal regions is explained as a resource partitioning strategy. This quite likely argues that although microzooplankton grazes strongly on phytoplankton in these regions, these microzooplankton grazers are passive.

Impact of microzooplankton and Calanus sinicus (Brodsky, 1962) on phytoplankton in the Yellow Sea during early summer

Dilution incubations and Calanus sinicus addition incubations were simultaneously conducted at five stations in the Yellow Sea in June of 2004 to evaluate the impact of microzooplankton and Calanus sinicus on phytoplankton based on the Chlorophyll a (Chl-a) levels. The Chl-a growth rates (k) ranged from 0.60-1.67 d(-1), while microzooplankton grazed the Chl-a at rates (g) of 0.29-0.62 d(t-1). The addition of C. sinicus enhanced the Chl-a growth rate (Z) by 0.004-0.037 d(-1) ind.(-1) L. C. sinicus abundance ranged from 84.1-160.9 ind. m(-3), which occupied 90.7%-99.1% of the copepod (> 500 mu m) population. The in-situ increase in phytoplankton by C. sinicus community was estimated to be 0.000 4-0.005 9 d(-1). These results showed that microzooplankton were the main grazers of phytoplankton, while C. sinicus induced a slight increase in the levels of phytoplankton.

Impact of microzooplankton and copepods on the growth of phytoplankton in the Yellow Sea and East China Sea

Dilution and copepod addition incubations were conducted in the Yellow Sea (June) and the East China Sea (September) in 2003. Microzooplankton grazing rates were in the range of 0.37-0.83 d(-1) stopin most of the experiments (except at Station A3). Correspondingly, 31-50% of the chlorophyll a (Chl a) stock and 81-179% of the Chl a production was grazed by microzooplankton. At the end of 24 h copepod addition incubations, Chl a concentrations were higher in the copepod-added bottles than in the control bottles. The Chl a growth rate in the bottles showed good linear relationship with added copepod abundance. The presence of copepods could enhance the Chl a growth at a rate (Z) of 0.03-0.25 (on average 0.0691) d(-1) ind(-1) l. This study, therefore parallels many others, which show that microzooplankton are the main grazers of primary production in the sea, whereas copepods appear to have little direct role in controlling phytoplankton.

中国近海浮游细菌生物量与生产力的生态分布特点

本论文研究了胶洲湾、东海和渤海的蓝细菌（Synechococcus）、生物量、异养细菌生物量和生产力的生态学特点。并在汇泉湾、渤海和东海用分极增减法对海洋蓝细菌在微型食物环（the microbial loop）中的作用进行了初步研究。在以上海区调查研究的时间如下：胶州湾：1993年2月、5月、9月11月，1996年5月、1999年3月、5月和12月。汇泉湾：1996年4月至1998年4月。东海：1997年2-3月，1998年7月。渤海1998年9-10月，1999年4-5月。研究结果如下：胶州湾：蓝细菌生物量的变化范围是11.4-0.03 mgC/m~3，季节变化是夏季>秋季和春季>冬季。其水平分布是除夏季蓝细菌生物量是沿岸浅水区向湾外递减外，其它三季（春、秋和冬季）是由湾外向湾内至沿岸浅水区递减。蓝细菌生物量与海水温度周年变化正相关，与季节海水温度的关系是秋、冬季分布变化一致，春、夏季分布变化相反。海水温度是影响胶州湾蓝细菌生物量分布变化的主要原因。异养细菌生物量和生产力的变化范围分别是29.8-1.62 mgC/m~3; 129.12-1.92 mgC/m~3.d。季节变化都是夏季>秋季、春季>冬季。夏季的异养细菌生物量和生产力水平分布趋势与蓝细菌生物量的分布变化相同。海水温度对异养细菌生产力的影响较对异养细菌生物量的影响大。异养细菌生产力相比（BP:PP）的变化在0.58-0.02之间，季节分布变化是夏季>秋季、春季>冬季。夏季表层的BP:PP由沿岸浅水区向湾心、湾口和湾外递减。东海：蓝细菌生物量的变化范围是46.72-0.011 mgC/m~3，夏季高平均是23.59 mgC/m~3，冬季低平均是3.61 mgC/m~3。冬季蓝细菌生物量的水平分布明显受黑潮的影响，在表面和20米层是由东南向西北方向递减。其垂直分布是冬季表层和20米层>底层，夏季是20米层>表层>底层；在连续站冬111站和410站变化都是中层>底层>表层。异养细菌生物量和生产力的变化范围分别是17.2-4.4 mgC/m~3（1997.2）；376.8-7.2 mgC/m~3.d。异养细菌生产力夏季高平均是35.1 mgC/m~3.d。异养细菌生物量的水平分布是由沿岸向外海递增（1997.2），异养细菌生产力的水平分布是冬季异养细菌生产力在32度断面有由沿岸向外递减趋势，PN断面的变化与冬季相似。垂直分布，冬季和夏季的异养细菌生产力的垂直变化在2断面是底层大于表面，PN断面则是表层大于底层，32度断面大多断站是底层大于表层。在连续站冬季111站异养细菌生产力的变化是底层>中层>表层，409站的变化是中层>底层>表层，夏季111站和410站都是中>底层>表层。异养细菌生物量（1997.2）表层分布变化与海水温度分布变化相似，底层变化相反。异养细菌生产力与初级生产力相比（BP:PP），冬季在0.04-0.30之间，平均为0.17；夏季在0.20-0.43之间平均0.32。冬季在长江口附近BP：PP有一个高值区是0.30，夏季在111站附近有一个高值区是0.43。从连续站111站和409’站观测发现底层的BP：PP明显高于表层。渤海：蓝细菌生物量秋季（16.6-0.37 mgC/m~3）比春季（0.44-0.015 mgC/m~3）高。其秋季的水平分布与海水盐度水平分布相同，与海水温度水平分布相反。异养细菌生产力秋季（189-62.2 mgC/m~3.d）与春季（193.2-49.8 mgC/m~3.d）相当。但秋季捕层BP普遍小于底层，而春季是表层普遍大于底层。根据颗粒分级培养实验结果，海洋蓝图细菌在微型食物环中的作用如下：在汇泉湾的春季和秋季蓝细菌可能主要被小型浮游动物（microzooplankton 20-200 μm）捕食。在渤海的春季和秋季也是同样结果。但在东海夏季的111站和410站附近（东海大陆架中部）微型浮游动物（nanozooplankton 2-20 μm）对蓝细菌的捕食压力明显。

东海大规模赤潮对微型浮游动物群落结构影响的研究

本文从赤潮发生前的现场模拟实验、赤潮发生时的现场培养实验和赤潮发生过程中的现场调查等方面，较为系统地研究了东海大规模赤潮对微型浮游动物群落结构的影响。 2005年长江口及邻近海区赤潮发生前，赤潮973MC2005－03航次在本海区进行了综合调查，期间分别于4月27日、5月4日和5月8日，在zzf1、zc18a和ra5三个站位利用现场船基培养的方法，研究了添加到赤潮密度106 cells L-1的东海原甲藻（Prorocentrum donghaiense）对微型浮游动物群落结构的影响。结果发现，赤潮密度的东海原甲藻对小型无壳纤毛虫的种群数量影响较大，而对中大型砂壳纤毛虫的影响较小，从而使得微型浮游动物群落有向中大型砂壳纤毛虫演替的趋势。在zzf1站位，小型无壳纤毛虫占绝对优势，添加东海原甲藻72 h后，优势种由管游虫（Cyrtostrombidium sp.）演替为另一种小型无壳纤毛虫急游虫4（Strombidium sp.4）；在以中型砂壳纤毛虫百乐拟铃虫（Tintinnospsis beroidea）为优势种的zc18a站位，仍然是以此为优势种，但其在群落中的优势度更加明显；ra5站位也是以小型无壳纤毛虫为主，添加东海原甲藻72 h后，其群落结构由急游虫2（Strombidium sp.2）向中大型砂壳纤毛虫纤毛虫百乐拟铃虫和亚速岛网纹虫（Favella azorica）演替。不同站位微型浮游动物群落结构变化的差异与其本身的群落结构组成有一定关系，也与浮游植物的群落组成有关。zzf1和ra5站位实验组中微型浮游动物的总丰度和总生物量都低于对照组，而zc18a站位变化不明显，这是由于前两个站位实验组中的优势种管游虫（zzf1站位）和急游虫2（ra5站位）的丰度和生物量迅速下降，而zc18a站位实验组的优势种百乐拟铃虫的丰度和生物量比较稳定造成的。 2005年长江口及邻近海区赤潮发生时，赤潮973 MC2005－04航次在本海区进行了综合调查，期间分别于5月26日、5月28日、5月28日和5月29日，在xzm1、srb、sra1和hb8a四个站位利用现场培养的方法，研究了东海大规模赤潮对微型浮游动物群落结构的影响。结果发现，东海大规模赤潮对微型浮游动物群落结构的影响与赤潮藻的密度密切相关：xzm1、srb和sra1站位是赤潮区，两种赤潮藻的总密度分别为，1.3×106 cells L-1、1.8×106 cells L-1和5.6×106 cells L-1，而hb8a站位位于非赤潮区，两种赤潮藻的总密度仅为5×105 cells L-1；实验进行72 h后，在以大型砂壳纤毛虫网纹虫和筒壳虫（Tintinnidium sp.）为主的xzm1、srb和sra1站位，仍然是以这些大型砂壳纤毛虫为主，且其在群落中的百分比显著增大，尤其在赤潮藻密度最高的的sra1站位，这种演替趋势表现的最明显；而在以小型无壳纤毛虫急游虫2为主的hb8a站位，微型浮游动物群落向中型砂壳纤毛虫真丁丁虫（Eutintinnus sp.）演替。培养过程中微型浮游动物总丰度的变化以及总生物量的变化都与赤潮藻的密度密切相关：赤潮藻密度较低的xzm1站位和srb站位变化不大；赤潮藻密度较高的sra1站位则快速下降；而非赤潮区的hb8a站位呈快速上升的趋势。以上结果进一步表明东海大规模赤潮能使微型浮游动物群落向中大型砂壳纤毛虫演替。 在2006年4月18日－5月30日，赤潮973项目MC2006航次在长江口及其邻近海区围绕东海原甲藻和米氏凯伦藻赤潮进行了综合调查。为了研究该过程中微型浮游动物群落结构的变化，我们在南北5个断面共计21个站位进行了取样，样品涉及赤潮发生前期、发生期和消退期。本次调查共发现纤毛虫66种，其中砂壳纤毛虫有8属37种，寡毛类无壳纤毛虫25种，另外还有前口类3属4种。在赤潮发生前之前，小型无壳纤毛虫的丰度普遍较高，平均值为1574 ind L-1，而中大型砂壳纤毛虫的丰度普遍偏低，平均值仅为14 ind L-1；赤潮发生以后，在大多数站位小型无壳纤毛虫的丰度出现不同程度的下降，平均值降为171 ind L-1，而中大型砂壳纤毛虫的丰度出现一定程度的上升，平均值增至216 ind L-1。赤潮消退时，小型无壳纤毛虫丰度的平均值降为109 ind L-1，而中大型砂壳纤毛虫的丰度继续增长，变为401 ind L-1。不同微型浮游动物百分比的统计结果也表明：微型浮游动物群落由小型无壳纤毛虫向中大型砂壳纤毛虫演替的趋势：小型无壳纤毛虫在群落中百分比的平均值由赤潮发生前的62 %变为赤潮发生时的32 %，至赤潮消退时又降为15 %；而中大型砂壳纤毛虫百分比的平均值从赤潮发生前的10 %变为赤潮发生时的24 %，至赤潮消退时增至50 %。典型站位za3、za5、zb7和zb9的结果再次表明了赤潮发生过程中微型浮游动物群落由小型无壳纤毛虫向中大型砂壳纤毛虫演替的趋势。 由以上赤潮发生前的现场模拟实验、赤潮发生时的现场培养实验和赤潮发生过程中的现场调查的结果均可见，东海大规模赤潮可以改变微型浮游动物的群落结构，呈现由小型无壳纤毛虫向中大型砂壳纤毛虫演替的趋势，进而有可能影响中大型浮游动物等摄食者的种群数量和群落结构，最终可能会影响整个海洋生态系统的结构和功能。

Experimental study on the impact of dinoflagellate Alexandrium species on populations of the rotifer Brachionus plicatilis

To investigate harmful effects of the dinoflagellate Alexandrium species on microzooplankton, the rotifer Brachionus plicatilis was chosen as an assay species, and tested with 10 strains of Alexandrium including one known non-PSP-producer (Alexandrium tamarense, AT-6). HPLC analysis confirmed the PSP-content of the various strains: Alexandrium lusitanicum, Alexandrium minutum and Alexandrium tamarense (ATHK, AT5-1, AT5-3, ATC102, ATC103) used in the experiment were PSP-producers. No PSP toxins were detected in the strains Alexandrium sp1, Alexandrium sp2. Exposing rotifer populations to the densities of 2000 cells ml(-1) of each of these 10 Alexandrium strains revealed that the (non-PSP) A. tarnarense (AT-6) and two other PSP-producing algae: A. lusitanicum, A. minutum, did not appear to adversely impact rotifer populations. Rotifers exposed to these three strains were able to maintain their population numbers, and in some cases, increase them. Although some increases in rotifer population growth following exposures to these three algal species were noted, the rate was less than for the non-exposed control rotifer groups. In contrast, the remaining seven algal strains (A. tamarense ATHK, AT5-1, AT5-3, ATC102, ATC103; also Alexandrium sp1 and Alexandrium sp2) all have adverse effects on the rotifers. Dosing rotifers with respective algal cell densities of 2000 cells ml-1 each, for Alexandrium spl, Alexandrium sp2, and A. tamarense strains ATHK and ATC103 showed mean lethal time (LT50) on rotifer populations of 21, 28, 29, and 36h, respectively. The remaining three species (A. tamarense strains AT5-1, AT5-3, ATC102) caused respective mean rotifer LT50S of 56, 56, and 71 h, compared to 160 h for the unexposed "starved control" rotifers. Experiments to determine ingestion rates for the rotifers, based on changes in their Chlorophyll a content, showed that the rotifers could feed on A. lusitanicum, A. minutum and A. tamarense strain AT-6, but could graze to little or no extent upon algal cells of the other seven strains. The effects on rotifers exposed to different cell densities, fractions, and growth phases of A. tamarense algal culture were respectively compared. It was found that only the whole algal cells had lethal effects, with strongest impact being shown by the early exponential growth phase of A. tamarense. The results indicate that some toxic mechanism(s), other than PSP and present in whole algal cells, might be responsible for the adverse effects on the exposed rotifers. (C) 2004 Elsevier B.V. All rights reserved.

Summer feeding activities of zooplankton in Prydz Bay, Antarctica

Grazing of dominant zooplankton copepods (Calanoides acutus. and Metridia gerlachei), salps (Salpa thompsoni) and microzooplankton was determined during the austral summer of 1998/1999 at the seasonal ice zone of the Prydz Bay region. The objective was to measure the ingestion rates of zooplankton at the seasonal ice zone, so as to evaluate the importance of different groups of zooplankton in their grazing impact on phytoplankton standing stock and primary production. Grazing by copepods was low, and accounted for less than or equal to 1% of phytoplankton standing stocks and 3.8-12.5% of primary production for both species during this study, even the ingestion rates of individuals were at a high level compared with previous reports. S. thompsoni exhibited a relatively high grazing impact on primary production (72%) in the north of our investigation area. The highest grazing impact on phytoplankton was exerted by microzooplankton during this investigation, and accounted for 10-65% of the standing stock of phytoplankton and 34-100% of potential daily primary production. We concluded that microzooplankton was the dominant phytoplankton consumer in this study area. Salps also played an important role in control of phytoplankton where swarming occurred. The grazing of copepods had a relatively small effect on phytoplankton biomass development.

Feeding activities of zooplankton in the Bohai Sea

The Bohai Sea was the site of the Chinese national GLOBEC programme. During the June 1997 cruises of R/V Science No.1, observations and experiments on zooplankton feeding were conducted. At five 48 h time-series stations the following observations and measurements on zooplankton were carried out: (1) diurnal vertical migration, by collecting samples at different layers every 3 h with a closing net; (2) diurnal feeding rhythms, by gut pigment analysis; and (3) ingestion rate, by both gut pigment analysis and the dilution method. A classification by body size was used to deal with the diversity of species and developmental stages of zooplankton assemblages. Samples were separated into three size groups: small (200-500 mu m), medium (500-1000 mu m) and large (> 1000 mu m). The results showed that the copepods (Calanus sinicus, Paracalanus parvus, Acartia bifilosa and Centropages mcmurrichi) performed clear diurnal vertical migrations. However, their behaviour was different at different stations. The variation in gut pigment content over the 24 h cycle showed strong diurnal feeding rhythms, particularly for the large size group. Gut pigment contents reached their daily maximum during the time from dusk to midnight (18:00-24:00). The peak value was about 10 times the minimum observed in the daytime. The in situ daily grazing rate, based on gut pigment contents and evacuation experiments, was 4.00-12.65 ng chla ind(-1) day(-1) for the small size group, 5.99-66.58 ng chla ind(-1) day(-1) for the medium size group and 31.31-237.13 ng chla ind(-1) day(-1) for the large size group. The copepods consumed only a small part (2.90-13.52%) of the phytoplankton biomass hut about 77% of the daily production. The grazing mortality rate of phytoplankton by microzooplankton (<200 mu m) measured by the dilution method ranged from 0.43 to 0.69 day(-1) The calculated daily consumption of phytoplankton biomass was 35-50%, and 85-319% of the potential production.