Ecological investigations of blooms of colonial Phaeocystis pouchetti--I. Abundance, biochemical composition, and metabolic rates

A winter bloom of the colonial stage of the prymnesiophyte Phaeocystispouchetit was studied in the 13-m³ mesocosms of the Marine EcosystemResearch Laboratory on Narragansett Bay, Rhode Island The tankswere temperature regulated at 4±2°C but differedin their nutrient concentrations and in situ irradiances. Oneof the tanks was a control without added nutrients, one receiveda temporary nutrient spike and two others received daily N/P/Siinputs. Photosynthesis and growth rates of colonies exposedto a range of natural light levels were measured at weekly intervals.Particulate carbon production and release of dissolved organiccarbon (DOC) by the entire plankton community was determinedconcurrently. Photosynthesis and growth rates of Phaeocystisin tanks receiving daily nutrient additions were asymptoticfunctions of irradiance. Light-saturated rates exhibited asymptoticrelationships with dissolved inorganic nitrogen (N) levels.N-Limited populations showed more variable responses. Althoughirradiance and N availability regulated the population dynamicsof Phaeocystis, the presence or absence of silicate (S₁) influencedits relative importance in each tank. Phaeocystis dominatedcommunity metabolism in the absence of Si, but co-occurred withextensive stands of diatoms when Si was available. A significantpositive correlation was found between the contribution by Phaeocystisto community production and the proportion of photosynthatereleased as DOC In all tanks, Phaeocystis populations exhibitedcycles of abundance in which division of cells within coloniespreceded the multiplication of colonies. The production of newcolonies apparently occurred via two mechanisms: the formationof colonies from solitary cells, and the cleavage of largercolonies into smaller daughter colonies. Phaeocystis in tankswith near undetectable nutrient levels contained C:N, C:Chla, and C:ATP ratios several times higher than colonies in nutnent-repletetanks. Phaeocystis C:Chl a and C:ATP ratios were substantiallygreater than those of non-gelatinous phytoplankton due to carbohydratestorage in colony gelatin In contrast, C:N ratios in Phaeocystisand non-gelatinous phytoplankton were similar, suggesting astorage depot of organic N outside of the cells. The resultssupport the notion that Phaeocystis colonies function as biologicalentities rather than as passive aggregations of cells.     

Comparative nutrient status and competitive interactions of two Antarctic diatoms (Corethron criophilum and Thalassiosira antarctica)

The nutrient status of two common Antarctic diatoms (Corethroncriophilum and Thalassiosira cf. antarctica) was analysed bystudying the growth response in enrichment bioassays and byestimates of the cell quotas of Si, N and P after size-fractionationof net plankton samples Corethron had higher biomass-specificN-quotas; Si- and P-quotas were quite similar between both species.Corethron was Si-limited in five enrichment experiments andnot nutrient limited in five experiments. Thalassiosira wasnot nutrient limited in six experiments, N-hmited in four experimentsand Si-limited in one experiment Droop-kinetics of Si-limitedgrowth of Corethron and of N-hmited growth of Thalassiosirawere obtained by combining the growth rates in the bioassaysand the cell quotas from the natural populations. The minimalSi-quota of Corethron was 0 041 mol Si/mol C, the saturatingcell quota was 0.158 mol Si/mol C respectively The minimal N-quotaof Thalassiosira was 0 0223 mol N/mol C, the saturating cellquota was 0.208 mol N/mol C Corethron was N-saturated at cellquotas of >0 106 mol N/mol C; Thalassiosira was Si-saturatedat cell quotas of <0 132 mol Si/mol C. As the better competitorfor N and the poorer one for Si, Corethron became more importantwith increasing Si N ratios, while Thalassiosira became moreimportant with decreasing Si:N ratios in the water     

Increase in Si:N drawdown ratio due to resting spore formation by spring bloom-forming diatoms under Fe- and N-limited conditions in the Oyashio region

Resting spore formation and Si:N drawdown ratios were investigated under iron (Fe)- and nitrogen (N)-limited conditions using a unialgal culture of Thalassiosira nordenskioeldii and natural phytoplankton assemblages during the spring bloom in the Oyashio region. In the unialgal culture of T. nordenskioeldii, 20% and 100% of the cells formed resting spores under Fe- and N-limited conditions, respectively. The Si:N drawdown ratios were 2- and 14-fold higher in Fe- and N-limited conditions, respectively, compared to Fe- and N-sufficient conditions. At the start of the natural phytoplankton incubation, 18 among 47 identified diatom species were known resting spore-forming species. Approximately 15 common diatom species formed resting spores under Fe- and N-limited conditions. During the natural phytoplankton incubation, the percentage of the resting spores increased with time under both Fe- and N-limited conditions, reaching 25% and 40% of total diatom abundance, respectively. The Si:N drawdown ratios significantly increased with an increase in the contribution of resting spores in both the unialgal culture and natural phytoplankton incubations. These results suggest that if the bloom dominated by neritic, resting spore-forming diatom species decline by either Fe- or N-depletion, Si may be utilized preferentially to N in the upper mixed layer due to the formation of heavily silicified resting spores.     

Copepod growth and diatoms: insensitivity of <Emphasis Type="Italic">Acartia tonsa</Emphasis> to the composition of semi-natural plankton mixtures manipulated by silicon:nitrogen ratios in mesocosms

The feeding selectivity and the growth and reproductive success of the copepod Acartia tonsa have been studied in mesocosms fertilized at different Si:N ratios (0–1.75:1) and, therefore, at different compositions of the phytoplankton communities. Phytoplankton composition showed a strong response to nutrient ratios, with diatoms comprising >90% at Si:N ratios >1:1 of total biomass as opposed to <20% at the lowest ratio. A. tonsa strongly preferred feeding on motile prey (flagellates and ciliates) to feeding on diatoms. Nevertheless, diatoms comprised a substantial part of the diet at the highest Si:N ratios. A. tonsa egg production and the final (after 4 weeks) abundance of adults and copepodites showed no response to Si:N ratios while nauplii production slightly increased with Si:N ratios. It is concluded that the frequently reported deleterious effect of diatoms on copepod reproduction is rather unusual when copepods are confronted with a naturally diverse phytoplankton assemblage instead of clonal cultures in the laboratory.     

The photosynthetic response of nutrient-depleted dilute cultures of Skeletonema costatum to pulses of ammonium and nitrate; the importance of phosphate

Experimental data on changes in carbon fixation rate causedby nutrient pulses in dilute cultures of nutrient-depleted Skeletonemacostatum are reported. Pulses contained single nutrients ormixtures of a nitrogen source and phosphate, concentrationsranging from 0 to 5 µM for ammonium and nitrate and from0 to 1 µM for phosphate. The cultures were incubated overnightafter pulsing to allow the rapid stage of nutrient uptake tocome to completion before measurement of carbon fixation thenext day. Increments in N:C ratios due to nitrogenous nutrientuptake depended upon the P:C ratio in the cells as well as theconcentration of the pulse. When P:C ratios were low (<0.005),increases in nitrogen repressed photosynthesis. Phosphate uptakewas independent of the absence or presence of a nitrogen sourceand had only a small stimulatory effect upon carbon fixation.When added jointly with ammonium or nitrate, however, largeincreases in the rate of photosynthesis resulted. These weredue mainly to rises in chlorophyll a concentrations resultingfrom higher N:C ratios in the cells. Chlorophyll-specific carbonfixation rates were hyperbolic functions of P:C ratios but exhibiteda C-shaped relationship to N:C ratios. The stimulatory and repressiveeffects of the nutrient pulses are shown to be consistent withthe view that the rate of photosynthesis at constant illuminationand carbon dioxide partial pressure is mainly controlled bythe chlorophyll a concentration and phosphate availability inthe phytoplankton.     

The species composition of Antarctic phytoplankton interpreted in terms of Tilman's competition theory

Summary An attempt was made, to test for the impact of resource competition on Antarctic marine phytoplankton. According to theory, species composition near competitive equilibrium should be determined by the ratios of limiting resources. Enrichment bioassays identified silicon and nitrogen as limiting nutrients for some of the most important phytoplankton species during early austral summer in the region near the Antarctic Peninsula. Together with the generally acknowledged limiting resource light, this gave three meaningful ratios of essential resources (Si:N, Si:light, N:light) and one ratio of substitutable resources (NO₃:NH₄). Phytoplankton species assemblages were found to be well separated by the ratios of the essential resources and by mixing depth. Nine out of 12 individual species were found to be separated along at least one of the gradients of resource ratios. Where comparison with competition experiments was available, predicted and realized distributions of species were compatible.     

Copepod and microzooplankton grazing in mesocosms fertilised with different Si:N ratios: no overlap between food spectra and Si:N influence on zooplankton trophic level

We hypothesized that the trophic level of marine copepods should depend on the composition of the protist community. To test this hypothesis, we manipulated the phytoplankton composition in mesocosms and measured grazing rates of copepods and mesozooplankton in those mesocosms. Twelve mesocosms with Northeast Atlantic phytoplankton were fertilised with different Si:N ratios from 0:1 to 1:1. After 1 week, ten of the mesocosms were filled with natural densities of mesozooplankton, mainly calanoid copepods, while two remained as mesozooplankton-free controls. Both before and after the addition of copepods there was a positive correlation of diatom dominance with Si:N ratios. During the second phase of the experiment, copepod and microzooplankton grazing rates on different phytoplankton species were assessed by a modification of the Landry-Hassett dilution technique, where the bottles containing the different dilution treatments were replaced by dialysis bags incubated in situ. The results indicated no overlap in the food spectrum of microzooplankton (mainly ciliates) and copepods. Ciliates fed on nanoplankton, while copepods fed on large or chain-forming diatoms, naked dinoflagellates, and ciliates. The calculated trophic level of copepods showed a significantly negative but weak correlation with Si:N ratios. The strength of this response was strongly dependent on the trophic levels assumed for ciliates and mixotrophic dinoflagellates.     

The relationship between carbon and biovolume in marine microbial mesocosms under different nutrient regimes

Nutrient manipulation experiments were conducted on a natural planktonic community in outdoor mesocosms. Inorganic nitrogen (N) and silicon (Si) were added to achieve N:Si ratios of 1:1 and 4:1. Total particulate carbon (PC) biomass of the microbial assemblage was determined by elemental analysis. Cell volume measurements by microscope on individual components of the community (bacteria, diatoms, photosynthetic nanoflagellates, heterotrophic nanoflagellates, dinoflagellates and ciliates) were also made. We applied published C:volume relationships to determine the volume estimated C content (CBV) of these microbial groups and hence of the total assemblage. The total CBV and total PC were compared to test the applicability of C:volume relationships under different nutrient regimes both before and after nutrient exhaustion. For initial N:Si ratios of 1:1, prior to nutrient exhaustion, the relationship between CBV and PC was linear with a gradient of approximately 1, (0.99?±?0.06), indicating that the published C:volume relationships accurately predicted the C content of the microbial assemblage. For N:Si ratios of 4:1, a linear relationship was again evident between CBV and PC (slope: 1.36?±?0.08). However, statistical comparison using a general linear model indicated that the gradient of this relationship differed significantly from that when the N:Si ratio was 1:1, and hence CBV overestimated elemental C. For both N:Si ratios, subsequent to nutrient exhaustion (N or Si), and hence when the diatom fraction of the microbial assemblage was in yield-limited post-exponential phase, the two measures of biomass were not well correlated. This indicated that measured cytoplasmic cell volume was a poor indicator of C biomass within the microbial assemblage in nutrient-deplete conditions.     

Carbon, nitrogen and phosphorus stoichiometry of crustacean zooplankton in the Baltic Sea: implications for nutrient recycling

The carbon (C), nitrogen (N) and phosphorus (P) contents (%of dry weight) of some crustacean zooplankton were studied inthe Baltic Sea. The copepod Acartia sp. had a stable C and Ncontent (48.3 ± 0.8% C, 12.4 ± 0.2% N, C:N ratio4.5 ± 0.1). The P content was variable (1–2%),probably depending on developmental stage and season. Copepodsaccumulating fat, like Pseudocalanus minutus elongatus, hadhigher and more variable C content (50–60%), and lowerN and P content (7–12% N, 0.6–1.5% P). The highestC and lowest N and P contents were found in adult Limnocalanusmacrurus. However, the N:P ratio was apparently independentof fat content and between 14 and 27 for all copepods. The cladoceransBosmina longispina maritima and Evadne nordmanni had lower Ncontent (9.3–10.8%) and higher C:N ratio (5.1–5.7)than Acartia sp. The P content (1.2–1.4%) was similarto Acartia sp. and the N:P ratios (16–19) were in thelower range of that found for the copepods. The N:P ratio wasgenerally somewhat higher in the copepods than in seston, whichmost of the year had nearly Redfield C:N:P ratios. Potentially,nutrient recycling from crustacean zooplankton could enhanceN limitation of phytoplankton, but small stoichiometric differencessuggest that this effect is probably weak. The extent is dependenton the structure of the zooplankton community and the grossgrowth efficiencies. Acartia copepodites, which had nearly RedfieldN:P ratios, would have the opposite effect and enhance P limitationin late summer when seston N:P ratios increased.     

广东大亚湾西南部海域营养盐结构的长期变化

分析了大亚湾西南部海域1982～2002年营养盐结构的长期变化。水域中溶解无机氮(DIN)的含量呈上升趋势,磷酸盐(PO₄^3-)含量从80年代初到90年代末逐年下降,2000年后有所恢复,硅酸盐(SiO₃^2-)的含量波动较大,但平均维持在20μmol·L^-1的较高水平上。各营养盐浓度比值的长期变化表现为氮磷比上升,硅氮比下降,硅磷比稳中有升。结合营养盐浓度及其比值的变化,可以认为大亚湾西南部海域中浮游植物生长的营养盐限制因子已经由过去的氮限制转变为磷限制;各营养盐的浓度及其比值在各季度的分布表明,浮游植物的生长春季和冬季受到氮限制的可能性较大,而夏季和秋季受磷限制的可能性较大;在营养盐的平面分布上,大鹏澳内DIN高于澳外水域,而PO₄^3-自1997年后低于澳外水域。     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

Does the chlorophyll a content of phytoplankton vary with trophic status in lakes on the New Zealand central volcanic plateau?

Recently, it has been shown that ratios of chlorophyll a toparticulate phosphorus (Chl a/PP) and chlorophyll a to particulatenitrogen (Chl a/PN) were significantly higher in eutrophic thanoligo/mesotrophic waters in 17 lakes on the central volcanicplateau, North Island, New Zealand. This difference was thoughtto be due to an increase in the chlorophyll a content of phytoplanktonin these eutrophic lakes. Corresponding measurements of chlorophylla and phytoplankton cell volume made during this study do notsupport this hypothesis. However, ratios of chlorophyll a toadenosine triphosphate and estimates of percentage phytoplanktonbiomass were significantly higher (P<0.05) in our eutrophicthan oligo/mesotrophic samples, suggesting that Chl a/PP andChl a/PN may be high in eutrophic waters simply because phytoplanktoncomprise more of the total microbial biomass. This hypothesisis supported by a strong linear relationship (r=0.88, P<0.001)between Chl a/PP and percentage phytoplankton biomass in sixof our study lakes where corresponding measurements were made.     

Seasonal phytoplankton responses to N:Si:P enrichment ratio in the Bizerte Lagoon (southwestern Mediterranean)

This study investigated the role of N, P and Si enrichments on phytoplankton in the Bizerte Lagoon (southwestern Mediterranean Sea, Tunisia) during March, June, August, October and December 2004. Polycarbonate bottles were enriched with different nutrients according to four treatments N:Si:P ratios [+NSi/-P (40:40:1), +P/-NSi (20:20:2,5), +NP/-Si (16:0:1) and +Si/-NP (16:32:1)] and incubated in situ during six days. Chl a and carbon biomass of phytoplankton varied significantly during the course of months, with the highest levels recorded in summer (4-4.4 microg Chl a L(-1) or 1126-1721 microg C L(-1)). Dinoflagellates dominated the initial phytoplankton communities, except in August, when diatoms represented a high fraction of microalgae (48%). Enrichment experiments induced significant increases in Chl a and in the final phytoplankton carbon biomasses. In summer (June/August), Si was the main limiting element for phytoplankton. Diatoms strongly responded to +Si/-NP and +NSi/-P enrichments and dominated the final phytoplankton communities (52-61%) in both treatments. Si played the most important role in the growth and development of diatoms. The biomasses and growth rates of dinoflagellates were significantly stimulated by +P/-NSi and +NP/-Si enrichments. After 6 days, dinoflagellates represented more than 70% of the total phytoplankton biomass in samples subjected to these treatments. Moreover, the addition of +P/-NSi increased the biomasses of several dinoflagellates. This suggests that dinoflagellates were mostly controlled by P availability. Unlike diatoms and dinoflagellates, flagellates showed weak responses to nutrient treatments during only some months of the year. The results showed that phytoplankton dynamics in the lagoon were influenced by nutrients in different manners.     

Species-specific differences in phytoplankton responses to N and P enrichments and the N:P ratio in the Archipelago Sea, northern Baltic Sea

A nutrient enrichment experiment was conducted in order to studythe role of nitrogen (N), phosphorus (P) and the N:P ratio onthe early summer phytoplankton community in the ArchipelagoSea, northern Baltic Sea. The phytoplankton community was, interms of chlorophyll a and total biomass, primarily N-limited,but the individual species varied in their responses to thenutrient supply. The recorded overall N limitation was due tofast growth responses of a few N-limited species such as thediatom Chaetoceros wighamii (Brightwell) and the mixotrophicchrysophyte Uroglena sp. Another dominating diatom, Skeletonemacostatum (Greville) Cleve was most clearly P-limited. The N:Pratio had the strongest effect on Uroglena sp., which grew exponentiallyin the enrichments with a high N:P ratio. This can be explainedby the ability of the species to feed on P-rich bacteria, whichgives it a competitive advantage in P-limited conditions. Thespecies-specific differences in the responses to the nutrientenrichments can generally be explained by differences in thespecies physiology and they were consistent with the theoryof resource competition.     

黄海和东海营养盐分布及其对浮游植物的限制

根据1997～1999年黄海和东海4个季节的现场调查资料,分析探讨了黄海、东海的营养盐分布特征及其对浮游植物生长的限制状况．结果表明,在长江口以东及其东北部海域终年存在一个范围很大的营养盐高值区．分析表明,这些营养盐主要来自长江冲淡水的扩展及苏北沿岸流的输送．此外,还获得了1998年长江流域特大洪水期间,迄今被观测到的长江冲淡水中营养盐的最大扩展范围,计算并研究了黄海、东海上层水中Si／N／P比值,结果表明,黄东海上层水中Si／N比值较高,Si不会成为黄东海浮游植物生长的限制因子;但在南黄海南部尤其是西南部、东海近岸及长江口以东海域,N／P比值很高(>30),说明与一般海洋环境的情况不同,黄海、东海有很大一部分海域浮游植物的生长受磷限制,而不是受氮限制．     

太平洋中西部海域浮游植物营养盐的潜在限制

2009年8月至9月期间在太平洋西部N1站和中部N2站进行现场营养盐加富培养实验。结果显示:N1站,浮游植物生物量对N或者P添加都有较强的响应,其中N+P+Si组和N+P组浮游植物长势迅速,叶绿素a从初始的0.03μg/L分别达到2.12μg/L和1.83μg/L,同时P先于N和Si之前被耗尽;说明N1站为N、P共同限制,P是首要限制因子。而N2站,浮游植物生物量仅对N、P共同添加有明显响应,N先于P和Si被浮游植物消耗殆尽。利用培养过程中营养盐比值变化推断,N1站浮游植物以低于Redfield ratio(16N∶1P)吸收N和P;而N2站浮游植物以高于Redfield ratio(16N∶1P)吸收N和P。这可能解释了太平洋西部的寡营养盐海域为潜在P限制,而在太平洋中部海域则为潜在N限制。     

Growth and bioluminescence of Noctiluca scintillans on varying algal diets

The effects of food concentration and food quality on the growthrate and bioluminescence potential of the heterotrophic dinoflagellateNoctiluca scintillans were examined. The growth rate of N. scinullansvaried greatly with the phytoplankton food it was provided,and those foods yielding higher growth rates also produced cellsthat were smaller in diameter, but with a greater bioluminescencepotential per unit volume. Small phytoplankton species, includingIsochrysis galbana and an unidentified chrysophyte responsiblefor the Texas brown tide, supported the lowest growth rates.Other small flagellates (Dunaliella tertiolecta) and dinoflagellates(Gyrodinium dorsum, Prorocentrum minimum) supported moderategrowth rates. The highest growth rates were supported by diatomsand prasinophytes. Maximum growth rates (0.5 day^-1) were obtainedwith the diatom Thalassiosira sp. at concentrations     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Effects of nitrogen enrichment on zooplankton biomass and N:P recycling ratios across a DOC gradient in northern-latitude lakes

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

     

Phytoplankton rate processes in the oligotrophic waters of the central North Pacific Ocean

The central North Pacific is one of the more oligotrophic regionsof the world oceans. There the particulate organic nitrogen:cabonratio of surface waters is variable and less than the Redfieldratio of 16N:106C by atoms. The phytoplankton P/B ratio basedupon both C and N assimilation rate varied directly with theparticulate matter PON:POC ratio as did the productivity index[mg C (mg chl a)^–1h^–1]. At steady state the doublingtime of the phytoplankton, the turnover time of the limitingnutrient supplied via herbivore grazing, and the time for herbivoresto filter a unit volume of water would be equivalent. They appearto be of the order of 5–9 days based on present methodologyand straightforward interpretation of its results. The rate measurements involved incubation of water samples forseveral hours in bottles. In the central N. Pacific the valueswere similar using bottles of different sizes. Addition of chelatorsdid not enhance the rates implying no poisoning of the planktonby heavy metal contaminants. The observed specific activitiesof ¹⁴C and ¹⁵N of the particulate matter in the rate measurementsare inconsistent with the notion of an active, rapidly growingand recycling microplankton food web within the incubation bottlesand support the idea that phytoplankton are growing slowly.