Size-fractionated productivity and nutrient dynamics of phytoplankton in subtropical coastal environments

It is now well established that the size distribution of phytoplankton plays an important role in primary production processes and nutrient dynamics of coastal environment. In situ observations showed that nanophytoplankton (3–20 μm) contributed 72.08% and58.18% of phytoplankton biomass and 58.32% and 41.14% of primary productivity to Xiamen Western Waters and the northern Taiwan Strait, respectively; picophytoplankton (0.2–3 μm) dominated the biomass (64.70%) and productivity (66.09%) in the southern Taiwan Strait. Furthermore, nanophytoplankton accounted for 75% of phosphate uptake with the highest rate constant (8.3×10^-5 s^-1) and uptake rate in unit water volume (5.4×10^-5 mmol dm^-3s^-1); picophytoplankton had the highest uptake rate in unit biomass (5.4×10^-5 mmol mg^-1s^-1) and photosynthetic index (3.8 mgC mgChl a^-1h^-1). All the results highlighted the remarkable characteristics of small size ranged (0.2–20 μm) phytoplankton in subtropical coastal environments: main contributor to phytoplankton biomass and production, high efficiency on organic carbon production and nutrient recycling. The far reaching environmental and ecological implications were discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Typha productivity in a Texas pond: Implications for energy and nutrient dynamics in freshwater wetlands

Above-and below-ground biomass of Typha angustifolia L. was sampled monthly for 18 months from a small Texas pond. Maximum above-ground biomass was 2559±284 g AFDW (ash-free dry weight) m⁻² in 1983 and 2895±217 g AFDW m⁻² in 1984. Peak below-ground biomass for these 2 years was 2506±278 g AFDW m⁻² and 2314±226 g AFDW m^t-2, respectively. Stepwise multiple linear regression analyses revealed that mean above-ground biomass accrual was related to duration of growing season, cumulative precipitation, cumulative degree days and/or cumulative pan evaporation. The same was not true for below-ground biomass increases. Analysis of covariance revealed that the rates of above-ground biomass production were not significantly different (F test, p < 0.05) between the 1983 and 1984 growing seasons. Below-ground biomass turnover times for 1983 and 1984 were 2.47 and 1.21 years, respectively.     

Carbon dioxide exchange and biomass productivity of the herbaceous layer of a managed tropical humid savanna ecosystem in western Kenya

Aims Humid savannas, as a result of high precipitation amounts, are highly productive. They are also hotspots for land use change and potential sources of carbon dioxide (CO₂) due to the large soil carbon (C) stocks. Understanding how ecosystem CO₂ exchange is influenced by changes arising from agricultural land use is vital in future management of these ecosystems and in responding to the ongoing shifts in management and climate. The aim of this study was to identify how ecosystem CO₂ exchange and biomass productivity of the herbaceous layer of a humid savanna in Kenya respond to current management practices.Methods We used flux chambers to quantify CO₂ fluxes, while monthly harvests were undertaken to determine biomass development of the herbaceous layer of three sites that were (i) fenced to exclude livestock grazing, (ii) subjected to grazing by livestock and (iii) abandoned after being cultivated for maize production and also open to grazing by livestock.Important findings The peak aboveground biomass ranged between 380 and 1449g m ?2 and biomass production was significantly (P < 0.05) lower in the grazed and abandoned plots. The maximum gross primary production (GPP) and net ecosystem CO₂ exchange (NEE) ranged between 21.8±1.3 to 32.5±2.7 and ?9.6±0.7 to^-17.9±4.8 μmol m ?2 s^-1, respectively. Seasonal NEE fluctuations ranged between 10 and 21 μmol m ?2 s^-1, while spatial (among sites) differences ranged between 2 and 10 μmol m ?2 s^-1. Ecosystem respiration (R eco) fluctuated between 5 and 10 μmol m ?2 s^-1 during the growing season. R eco was, however, not significantly different among the sites. Unlike in other similar ecosystems where ecosystem respiration is determined by the ambient temperature, we did not find any relationship between R eco and temperature in this savanna. Instead, soil moisture accounted for 38–88% of the spatial and seasonal fluctuations in ecosystem CO₂ fluxes and aboveground biomass production. Management influenced the maximum GPP and NEE rates through modification of soil moisture, plant species composition and aboveground biomass. We concluded that soil moisture is the key determinant of ecosystem CO₂ exchange and productivity in this tropical savanna. Management, however, significantly modifies C fluxes and productivity through its influence on soil moisture, plant species composition and aboveground green biomass and should be taken into consideration in future similar studies.     

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha^?1 season^?1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha^?1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to ?0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg^?1 N, 558 kg DM kg^?1 P and 111 kg DM kg^?1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha^?1 and 30 kg P ha^?1 + 90 kg N ha^?1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.     

Foliar nutrient dynamics and nutrient use efficiency in Cornus florida

Summary Growth rates and seasonal changes in foliar nitrogen, phosphorus, and calcium of Cornus florida L. (flowering dogwood) individuals were determined in three forest stands which differed in soil moisture and soil nutrient availability. Nutrient use efficiency of individual trees was measured by amount of leaf dry mass produced per unit nutrient invested, rates of nutrient resorption prior to litterfall, wood and leaf mass produced per unit nutrient turnover (=growth efficiency), projected uptake needs, and losses of nutrients to simulated throughfall leaching. Relative growth rates during this drought year, as determined by dimension analysis, were highest in the site with highest soil moisture, while 5-year average relative growth rates were highest in the most fertile site. Differences in nitrogen use efficiency were generally small, with the highest efficiencies in trees on the moistest site; in contrast, phosphorus use efficiency was consistently highest on the least fertile site. Foliar calcium levels increased throughout the year and calcium use efficiency was generally highest on the least fertile site. These data suggest that growth and nitrogen use efficiency were more strongly affected by differences in soil moisture than were phosphorus or calcium use, at least during this very dry year.     

Trophic state and nutrient limitation in Lake Baringo,Kenya

The trophic state of Lake Baringo and factors that could be limiting the development of algal biomass in it were investigated during one wet/dry hydrological cycle in 2014–2015. Water samples were analysed for dissolved inorganic nutrients, including , and , total phosphorus and Chlorophyll a. Light attenuation was estimated using Secchi depth. The trophic state was determined using Carlson trophic state indices (CTSI). Deviations in CTSI, nutrient ratios and ambient nutrient concentrations were used to identify factors limiting phytoplankton growth. The mean values measured for Secchi depth, nitrate, total phosphorus and Chlorophyll a showed significant seasonal variation (p < 0.05). Based on the Carlson trophic state index, the results show that Lake Baringo is eutrophic. However, the lake is also experiencing phosphorus limitation and poor light penetration, because of high turbidity, which is more pronounced during the wet season.     

连栽第1和第2代杉木人工林养分循环的比较

森林生态系统的养分循环是生态系统的重要功能过程之一,直接影响着森林的生产力,很大程度上影响和制约着林地的肥力水平,而且人工林连栽地力衰退和生产力下降现象普遍存在,寻求杉木林连栽两代杉木人工林养分循环差异与连栽林分生产力下降的关系,无疑具有重要的现实意义。利用30多年连续定位的测定数据,分析了连栽第1、2代杉木人工林在物质生产养分利用有效性、生物地球化学循环、地球化学循环的差异。结果表明,杉木速生阶段,第2代林每生产1 t干物质需要的养分比第1代林多1.58-3.29 kg,干材生长阶段,第2代林每生产1t所需养分比第1代林多4.23-5.92kg;速生阶段生物地球化学循环的养分利用系数第2代林比第1代林分下降19.7%-22.8%,养分循环系数下降12.8%-15.6%,干材生长阶段养分利用系数比第1代林分下降35.3%-36.2%,养分循环系数下降23.2%-27.0%,养分周转利用的生物地球化学循环功能第2代林比第1代林差;由干材生长进入成熟阶段的生长期内,伴随水文学过程的养分地球化学循环中,第2代杉木人工林生态系统的养分积累的地球化学循环的能力下降,养分流失率是第1代林的2倍左右,养分的积累率还不到第1代林的60%,对森林地力的维持和林木生长都是不利的。从生态系统水平上定位研究,定量分析了杉木连栽两代人工林养分循环功能过程,研究成果为我国南方人工林持续经营措施的制定提供了理论指导和科学依据。     

Bryozoan populations reflect nutrient enrichment and productivity gradients in rivers

1. The hypothesis that nutrient enrichment will affect bryozoan abundance was tested using two complementary investigations; a field‐based method determining bryozoan abundance in 20 rivers of different nutrient concentrations by deploying statoblast (dormant propagule) traps and an experimental laboratory microcosm study measuring bryozoan growth and mortality. These two methods confirmed independently that increased nutrient concentrations in water promote increases in the biomass of freshwater bryozoans. 2. Statoblasts of the genus Plumatella were recorded in all rivers, regardless of nutrient concentrations, demonstrating that freshwater bryozoans are widespread. Concentrations of Plumatella statoblasts were high in rivers with high nutrient concentrations relative to those with low to moderate nutrient concentrations. Regression analyses indicated that phosphorus concentrations, in particular, significantly influenced statoblast concentrations. 3. Concentrations of Lophopus crystallinus statoblasts were also higher in sites characterised by high nutrient concentrations. Logistic regression analysis revealed that the presence of L. crystallinus statoblasts was significantly associated with decreasing altitude and increasing phosphorus concentrations. This apparently rare species was found in nine rivers (out of 20), seven of which were new sites for L. crystallinus. 4. Growth rates of Fredericella sultana in laboratory microcosms increased with increasing nutrient concentration and high mortality rates were associated with low nutrient concentrations. 5. Our results indicate that bryozoans respond to increasing nutrient concentrations by increased growth, resulting in higher biomasses in enriched waters. We also found that an important component of bryozoan diets can derive from food items lacking chlorophyll a. Finally, bryozoans may be used as independent proxies for inferring trophic conditions, a feature that may be especially valuable in reconstructing historical environments by assessing the abundance of statoblasts in sediment cores.     

Malaria vectors in Lake Victoria and adjacent habitats in western Kenya

The prevalence of malaria among the residents of the Lake Victoria basin remains high. The environment associated with the lake may maintain a high number of malaria vectors. Lake habitats including water hyacinths have been suspected to be the source of vectors. This study investigated whether malaria vectors breed in the lake habitats and adjacent backwater pools. Anopheline larvae were collected within the littoral zone of the lake and adjacent pools located along approximately 24.3 km of the lakeshore in western Kenya, and their breeding sites characterized. Three primary vector species, Anopheles arabiensis, Anopheles gambiae s.s. and Anopheles funestus s.s., and three potential vectors, were found in the lake habitats. Unexpectedly, An. arabiensis was the most dominant vector species in the lake sampling sites. Its habitats were uncovered or covered with short grass. A potential secondary malaria vector, Anopheles rivulorum, dominated the water hyacinths in the lake. Most breeding sites in the lake were limited to areas that were surrounded by tall emergent plants, including trees, and those not exposed to waves. Nearly half of adjacent habitats were lagoons that were separated from the lake by sand bars. Lagoons contained a variety of microhabitats. Anopheles arabiensis dominated open habitats, whereas An. funestus s.s. was found mainly in vegetated habitats in lagoons. The current study confirmed that several breeding sites are associated with Lake Victoria. Given that Lake Victoria is the second largest lake in the world, the lake related habitats must be extensive; therefore, making targeted vector control difficult. Further exploration is necessary to estimate the effects of lake associated habitats on malaria transmission so as to inform a rational decision-making process for vector control.     

Biomass dynamics of grassland vegetation in Kenya

Seasonal changes in plant biomass in the herb layer were measured at Nairobi National Park and Masai Mara Game Reserve in Kenya from January 1980 to February 1981. Plant biomass fluctuated in response to seasonal rainfall, and live biomass was correlated with rainfall and soil moisture at both locations. Peak values for live biomass at Nairobi National Park occurred at the end of the long rains during the June 1980 sample and ranged from 138 to 197gm^-2. Minimum values for live biomass at Nairobi occurred during a dry season in February 1981 and ranged from 8 to 39 g m^-2. The standing dead and litter compartments were larger than the live compartment during every sample period at Nairobi National Park, and together often comprised more than 80% of the total above-ground biomass. Probably as a result of higher rainfall, peak values for live biomass at Masai Mara Game Reserve were higher than those at Nairobi. Again, peak biomass occurred during June following the long rains, and ranged from 218 to 294 gm^-2. Minimum values for live biomass occurred during February 1981, and ranged from 10 to 48 g m^-2. Standing dead and litter compartments were much smaller than at Nairobi National Park, reflecting more intense herbivore pressure and a greater frequency of fires at Masai Mara compared with that at Nairobi National Park.     

Beyond the everyday: sustaining kinship in western Kenya

This article considers the efforts people in western Kenya have been making to uphold an ideology and practice of the natal home and kin group as morally authoritative, in a context where the very survival of many homes and families has been compromised by the devastating effects of AIDS‐related deaths and impoverishment. It traces how orphaned adults, who have little experience or memory of living among natal kin at natal homes, make concerted efforts to reconnect – often in necessarily improvised ways – with what survives of their natal kin and home. For women, in particular, such efforts seem less motivated by immediate material interests and more focused on demonstrating lineal solidarity as a means to affirming their moral personhood and value. The analysis addresses how people lacking shared everyday experiences of kinship and homes sustain the possibility of their kinship futures through a combination of imagination and ideological commitment.     

辽西人工林气候生产力分析

采用6种方法计算了辽西地区的人工林气候生产力。结果表明,辽西平均人工林生产潜力为8．8t·hm^-2．年^-1,其分布趋势从东南向西北逐渐降低,与降雨量分布一致。针对地区特点,生态地区法、层次递推法和综合模型是适宜的气候生产力计算模型。对比分析表明。辽西现实人工林生产力水平处于极低水平。提高人工林质量,挖掘生产潜力是今后人工林发展的关键．     

Carbon losses and primary productivity decline in savannah soils under cotton-cereal rotations in semiarid Togo

Soil degradation in the savannah-derived agroecosystems of West Africa is often associated with rapid depletion of organic carbon stocks in soils of coarse texture. Field experiments were conducted over a period of more than 30 years at two sites in semiarid Togo to test the impact of agricultural management practices on soil C stocks and crop productivity. The resulting datasets were analysed using dynamic simulation models of varying complexity, to study the impact of crop rotation, fertiliser use and crop residue management on soil C dynamics. The models were then used to calculate the size of the annual C inputs necessary to restore C stocks to thresholds that would allow positive crop responses to fertilisers under continuous cultivation. Yields of all crops declined over the 30 years irrespective of crop rotation, fertiliser use or crop residue management. Both seed-cotton and cereal grain yields with fertiliser fluctuated around 1 t ha^?1 after 20 years. Rotations that included early maturing sorghum varieties provided larger C inputs to the soil through residue biomass; around 2.5 t C ha^?1?year^?1. Soil C stocks, originally of 15 t ha^?1 after woodland clearance, decreased by around 3 t ha^?1 at both sites and for virtually all treatments, reaching lower equilibrium levels after 5–10 years of cultivation. Soil C dynamics were well described with a two-pool SOM model running on an annual time step, with parameter values of 0.25 for the fraction of resistant plant material (K₁), 0.15–0.20 for the decomposition rate of labile soil C (K₂) and 8–10 t C ha^?1 for the fraction of stable C in the soil. Simulated addition of organic matter to the soil 30 years after woodland clearance indicated that additions of 3 t C ha^?1?year^?1 for 15–20 years would be necessary to build ‘threshold’ soil C stocks of around 13 t ha^?1, compatible with positive crop response to fertiliser. The simulated soil C increases of 0.5 to 1.6% per year are comparable with results from long-term experiments in the region. However, the amounts of organic matter necessary to build these soil C stocks are not readily available to resource-poor farmers. These experimental results question the assumption that crop residue removal and lack of fertiliser input are responsible for soil C decline in these soils. Even when residues were incorporated and fertilisers used at high rates, crop C inputs were insufficient to compensate for C losses from these sandy soils under continuous cultivation.     

Recent papyrus swamp habitat loss and conservation implications in western Kenya

Papyrus Cyperus papyrus swamps are characteristic of many wetlands of tropical Africa. Like most wetland habitats worldwide, they are under human pressure due to harvesting and reclamation for agriculture. Changes in papyrus cover were assessed using aerial photographs at three Important Bird Areas (IBAs) in the Kenyan sector of Lake Victoria. Papyrus area losses of 50% (Dunga), 47% (Koguta) and 34% (Kusa) occurred between 1969 and 2000. The habitat loss and degradation at the sites appeared purposeful, driven by demand for agricultural land and other papyrus products. Cleared papyrus and cultivated areas around all sites increased over the same period. If papyrus habitat loss continues at current rates, papyrus swamps at Dunga and Koguta will disappear by 2020, and Kusa will be reduced to only 19% of its 1969 area. Human population growth around the sites, with concomitant increase in land use activities and papyrus harvesting are the major factors that account for papyrus area reductions. Loss of total habitat is accompanied by deterioration in habitat quality within remaining areas. Papyrus physical structure (height and density) inversely correlate to human disturbances that include footpaths, cutting, burning, grazing and farming. The continued papyrus habitat loss and degradation represents a significant threat to biodiversity conservation particularly for papyrus-specialist birds and other papyrus-reliant species in western Kenya. The observed pattern of papyrus extents and land use changes at all sites provide the site-scale information necessary for papyrus conservation planning. In particular, conservation action is needed most urgently at Dunga and Koguta as they face severe land use pressures.     

Introgressive hybridization of human and rodent schistosome parasites in western Kenya

Hybridization and introgression can have important consequences for the evolution, ecology and epidemiology of pathogenic organisms. We examined the dynamics of hybridization between a trematode parasite of humans, Schistosoma mansoni, and its sister species, S. rodhaini, a rodent parasite, in a natural hybrid zone in western Kenya. Using microsatellite markers, rDNA and mtDNA, we showed that hybrids between the two species occur in nature, are fertile and produce viable offspring through backcrosses with S. mansoni. Averaged across collection sites, individuals of hybrid ancestry comprised 7.2% of all schistosomes collected, which is a large proportion given that one of the parental species, S. rodhaini, comprised only 9.1% of the specimens. No F1 individuals were collected and all hybrids represented backcrosses with S. mansoni that were of the first or successive generations. The direction of introgression appears highly asymmetric, causing unidirectional gene flow from the rodent parasite, S. rodhaini, to the human parasite, S. mansoni. Hybrid occurrence was seasonal and most hybrids were collected during the month of September over a 2-year period, a time when S. rodhaini was also abundant. We also examined the sex ratios and phenotypic differences between the hybrids and parental species, including the number of infective stages produced in the snail host and the time of day the infective stages emerge. No statistical differences were found in any of these characteristics, and most of the hybrids showed an emergence pattern similar to that of S. mansoni. One individual, however, showed a bimodal emergence pattern that was characteristic of both parental species. In conclusion, these species maintain their identity despite hybridization, although introgression may cause important alterations of the biology and epidemiology of schistosomiasis in this region.     

Plio-Pleistocene cercopithecids from Kanam East, western Kenya

This paper describes cercopithecid craniodental and postcranial fossils recovered by L. S. B. Leakey at Kanam East, Kenya during the early 1930s. These fossil monkeys have been generally assumed to have been derived from early Pliocene horizons, but their exact geographical and stratigraphical provenience is unknown. Although the question of the evolutionary significance of these specimens must await the recovery of more securely dated material from Kanam East, some general conclusions can be drawn concerning their taxonomic affinities and paleobiology. Based on comparative studies of the craniodental material, at least three extant genera are represented—Colobus,Lophocebus, andCercopithecus. The postcranial fossils include a number of hindlimb specimens, as well as the manubrium of a sternum and a caudal vertebra. Identification of the postcranial remains to particular genera is not possible, but they are similar in morphology to modern arboreal and semiterrestrial cercopithecid monkeys of small to medium size. It is evident that Kanam East had a diverse cercopithecid community, similar to those found today in forested and woodland habitats, and this may be of some significance in reconstructing the paleoecology of the site. Because the fossil record of most extant cercopithecid genera is rather sparse at Plio-Pleistocene sites in Africa, Kanam East represents one of only a few sites that has yielded material that can be assigned toColobus,Lophocebus, orCercopithecus. The fossil monkeys from the site, therefore, provide additional evidence to help reconstruct the paleobiology, as well as the patterns of species diversity and community structure that characterized the cercopithecid radiation during the Plio-Pleistocene.     

Primary productivity,phytoplankton and limiting nutrient factors in labrador lakes

The primary productivity of some lakes and reservoirs in western Labrador was measured by the ¹⁴C method in order to determine the range of productivities and the effects of impoundment. No primary productivity data previously existed for this part of Canada. Both the primary productivity and standing crops of phytoplankton were found to be low in a newly impounded lake but later rose to levels greater than in surrounding natural lakes. In nutrient enrichment experiments, carbon was never found to be limiting but phosphorus stimulated primary productivity when added alone or in combination with nitrogen.     

Effects of resource pulse magnitude on nutrient availability,productivity, stability,and food web dynamics in experimental aquatic ecosystems

Oceanographic processes play a key role in influencing the structure of the marine planktonic ecosystems. Taking advantage of the quasi-simultaneous collection of a large ichthyoplanktonic dataset in different regions of the Central Mediterranean Sea (Italian/Maltese, Tunisian and Libyan waters), this study aimed at the identification of the main environmental drivers that control the structure of the larval fish assemblages. Spatial distribution and taxa composition were related to physical forcings (geostrophic currents and wind stress) and environmental conditions (bottom depth, temperature, salinity, chlorophyll-a concentration). ANOSIM and SIMPER identified contribution of fish taxa to the average Bray–Curtis dissimilarity among regions. In Italian and Libyan waters, two assemblages (neritic and oceanic) were identified, while a mixed assemblage characterized only some stations. Two neritic and one oceanic assemblages were discriminated in Tunisian waters. Random Forest classification model highlighted the essential role of the bathymetry, while Lagrangian simulations evidenced the action of the hydrodynamics in mixing neritic and oceanic assemblages in the Italian/Maltese and partially in Libyan waters. These findings highlighted the importance of the multidisciplinary approach and shed light on the potential value of the ichthyoplanktonic surveys for the assessment of the state of the marine ecosystem and the conservation of the fishery resources.     

Thermal stratification, nutrient dynamics, and phytoplankton productivity during the onset of spring phytoplankton growth in Lake Baikal, Russia

Lake Baikal, Russian Siberia, was sampled in July 1990 during the period of spring mixing and initiation of thermal stratification. Vertical profiles of temperature, dissolved nutrients (nitrate and soluble reactive phosphorus), phytoplankton biomass, and primary productivity were determined in an eleven-station transect encompassing the entire 636 km length of the lake. Pronounced horizontal variability in hydrodynamic conditions was observed, with the southern region of the lake being strongly thermally stratified while the middle and north basins were largely isothermal through July. The extent of depletion of surface water nutrients, and the magnitude of phytoplankton biomass and productivity, were found to be strongly correlated with the degree of thermal stratification. Horizontal differences likely reflected the contribution of two important factors: variation in the timing of ice-out in different parts of the lake (driving large-scale patterns of thermal stratification and other limnological properties) and localized effects of river inflows that may contribute to the preliminary stabilization of the water column in the face of intense turbulent spring mixing (driving meso-scale patterns). Examination of the relationships between surface water inorganic N and P depletion suggested that during the spring and early summer, phytoplankton growth in unstratified portions of the lake was largely unconstrained by nutrient supplies. As summer progressed, the importance of co-limitation by both N and P became more apparent. Uptake and regeneration rates, measured directly using the stable isotope ¹⁵N, revealed that phytoplankton in stratified portions of the lake relied primarily on NH₄ as their N source. Rates of NH₄ regeneration were in approximate equilibrium with uptake; both processes were dominated by organisms <2 µm. This pattern is similar to that observed for oligotrophic marine systems. Our study underscores the importance of hydrodynamic conditions in influencing patterns of biological productivity and nutrient dynamics that occur in Lake Baikal during its brief growing season.