The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the relative balance of light and phosphorus in a set of freshwater pelagic ecosystems. We calculated a ratio of light: phosphorus by putting mixed-layer mean light in the numerator and total P concentration in the denominator. This light: phosphorus ratio was a good predictor of the C:P ratio of particulate matter (seston), with a positive correlation demonstrated between these two ratios. We argue that the balance between light and nutrients controls "nutrient use efficiency" at the base of the food web in lakes. Thus, when light energy is high relative to nutrient availability, the base of the food web is carbon rich and phosphorus poor. In the opposite case, where light is relatively less available compared to nutrients, the base of the food web is relatively P rich. The significance of this relationship lies in the fact that the composition of sestonic material is known to influence a large number of ecosystem processes such as secondary production, nutrient cycling, and (we hypothesize) the relative strength of microbial versus grazing processes. Using the central result of increased C:P ratio with an increased light: phosphorus ratio, we make specific predictions of how ecosystem structure and process should vary with light and nutrient balance. Among these predictions, we suggest that lake ecosystems with low light: phosphorus ratios should have several trophic levels simultaneously carbon or energy limited, while ecosystems with high light: phosphorus ratios should have several trophic levels simultaneously limited by phosphorus. Our results provide an alternative perspective to the question of what determines nutrient use efficiency in ecosystems.

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Planktonic bacteria may represent a substantial nutrient resource available to support the next higher trophic level in the microbial loop (heterotrophic flagellates). In this work we examined the utilization of different size classes of bacteria by flagellated protozoan predators of various sizes. The emphasis was to determine if prey-size selection was a function of predator size. Pseudomonas sp. was grown in chemostats under conditions to yield "large" (mean size, 1.19 pm') or "small" individuals (mean size, 0.36 pm3). Cells were fluorescently labeled (DTAF), mixed in various proportions, and fed to four protozoans ranging in size from 21 to 119 pm3. Bacteria in food vacuoles were enumerated and measured. Size distributions of both offered and taken prey were compared, and a selectivity index was calculated. Protozoans of all four sizes preferred large bacteria (between 0.8 and 1.2 pm').

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Extensive measurements of material concentrahons and water velocities at a transect across North Inlet, South C a r o h a (USA) allowed the estimation of net material and water fluxes. Sampling periods were distnbuted seasonally and according to tidal height. Statistical and hydrodynanlic models were used to develop flux estimates for specific tidal cycles. There was a net discharge of water from the marsh-estuanne system to the Atlantic Ocean which is attributed to rainfall runoff and freshwater input from an adjacent estuary. All constitutents were exported seasonally and annually from the system -except total semments, imported during fall and winter, and chlorophyll a and zooplankton, imported in summer and fall. ATP, bird biomass and macrodetritus were exported throughout the year. Export of carbon, nitrogen and phosphorus from North Inlet is high compared to other systems studied to date. Large net fluxes of DOC during winter imply coupling with the uplands. The high rates of export of ammonium and orthophosphate along with detritus and microorganisms suggest major decomposition processes are taking place within the system. Export of ammonium and orthophosphate to the coastal ocean also suggest a feedback loop with phytoplankton utilizing these materials, then in turn phytoplankton are imported into the estuary where they are consumed and remineralized. Previous explanations of outwehng and tidal transport are examined and found to be individually lacking. It is proposed that any comprehensive explanahon of the magnitude and direction of transport must include a physical explanation of water motion and the biological and physical mechanisms by which materials are added or removed from tidal waters. A comprehensive explanation of outwelling in North Inlet IS proposed which describes this system as an ebb-dominated, bar-built estuary with good flow connection to the sea and with some freshwater input. It is also a fertile system with high productivity providing materials for export to the coastal ocean and utilization of other imported materials.

Size-selective grazing of three heterotrophic nanoflageliates (with cell sizes of 21, 44, and 66 jim3) isolated from Lake Arlington, Texas was examined by using a natural mixture of fluorescence labelled lake bacteria. Sizes of ingested bacteria in food vacuoles were directly measured. Larger bacterial cells were ingested at a frequency much higher than that at which they occurred in the assemblage, indicating preferential flagellate grazing on the larger size classes within the lake bacterioplankton. Water samples were collected biweekly from June through September, 1989, fractionated by filtration, and incubated for 40 h at in situ temperatures. The average bacterial size was always larger in water which was passed through 1-jLm-pore-size filters (1-jimfiltered water) (which was predator free) than in 5-jLm-filtered water (which contained flagellates only) or in unfiltered water (in which all bacterivores were present). The increase of bacterial-cell size in 1-jm-filtered water was caused by a shift in the size structure of the bacterioplankton population. Larger cells became more abundant in the absence of flagellate grazing.

Bactena play a sign~ficant role in the nutnent dynamics of plankton~c systems, yet the11 nutrient demands are poorly known Heie, we charactenze the element latios of a common freshwater bactenum relative to growth rate and nutrient availability P s e~~d o m o n a s fluorescens was grown in chemostats at dilution rates of 0 03 0 06, and 0 09 h ' Phosphorus (P) was supplied at approximately 5 ~.IM and nitrogen (N) was supphed at varylng concentlatlons to establish resource molar-N P ratios (SNP) ranglng between 5 1 and 117 1 Carbon was supplied In excess Chemostats were sampled at steady state the element composition of cells (Q) was determined and element ratlos (C& , ) w e l e calculated QL was 7 6 l and varied little with respect to growth rate or the element composltlon of the medium In contrast Q N p was varlable and positively related to the SNP until an upper h m~t of -21 1 was attained at a SN p of 33 1 QC p mimicked QN P and was constrained by the flxed QC , , P fluorescens appeared to have a high capaclty to accumulate P a n d the data suggested that the organism may be capable of ~ncorporating P above that required to meet metabolic demand ('luxury uptake ) Comparison of these data with previously published data revealed that the bactena have a much greater potential to adjust the11 QNp when the N P ratio of iesources supporting growth is low ( N scarce) than when the ratio is hlgh (N abundant) The C N P ratio of bacteria was found to vary between 5 2 8 1 when N was scarce ~e l a t l v e to P ( N P < 40 1) to as h~g h as 163 25 1 when N was abundant relative to P (N P > 40 1)

BackgroundReproductive isolation is a defining characteristic of populations that represent unique biological species, yet we know very little about the gene expression basis for reproductive isolation. The advent of powerful molecular biology tools provides the ability to identify genes involved in reproductive isolation and focuses attention on the molecular mechanisms that separate biological species. Herein we quantify the sterility pattern of hybrid males in African Clawed Frogs (Xenopus) and apply microarray analysis of the expression pattern found in testes to identify genes that are misexpressed in hybrid males relative to their two parental species (Xenopus laevis and X. muelleri).Methodology/Principal FindingsPhenotypic characteristics of spermatogenesis in sterile male hybrids (X. laevis x X. muelleri) were examined using a novel sperm assay that allowed quantification of live, dead, and undifferentiated sperm cells, the number of motile vs. immotile sperm, and sperm morphology. Hybrids exhibited a dramatically lower abundance of mature sperm relative to the parental species. Hybrid spermatozoa were larger in size and accompanied by numerous undifferentiated sperm cells. Microarray analysis of gene expression in testes was combined with a correction for sequence divergence derived from genomic hybridizations to identify candidate genes involved in the sterility phenotype. Analysis of the transcriptome revealed a striking asymmetric pattern of misexpression. There were only about 140 genes misexpressed in hybrids compared to X. laevis but nearly 4,000 genes misexpressed in hybrids compared to X. muelleri.Conclusions/SignificanceOur results provide an important correlation between phenotypic characteristics of sperm and gene expression in sterile hybrid males. The broad pattern of gene misexpression suggests intriguing mechanisms creating the dominance pattern of the X. laevis genome in hybrids. These findings significantly contribute to growing evidence for allelic dominance in hybrids and have implications for the mechanism of species differentiation at the transcriptome level.