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The effects of nutrient pollution on coral reef ecosystems are multifaceted. Numerous experiments have sought to identify the physiological effects of nutrient enrichment on reef‐building corals, but the results have been variable and sensitive to choices of nutrient quantity, chemical composition and exposure duration.To test the effects of chronic, ecologically relevant nutrient enrichment on coral growth and photophysiology, we conducted a 5‐week continuous dosing experiment on two Hawaiian coral species, Porites compressa and Pocillopora acuta. We acclimated coral fragments to five nutrient concentrations (0.1–7 µM NO3‐ and 0.06–2.24 µM PO43‐) with constant stoichiometry 2.5:1 nitrate to phosphate) bracketing in situ observations from reefs throughout the Pacific.Nutrient enrichment linearly increased photophysiological performance of both species within 3 weeks. The effect of nutrients on P. acuta photochemical efficiency increased through time while a consistent response in P. compressa indicated acclimation to elevated nutrients within 5 weeks. Endosymbiont densities and total chlorophyll concentrations also increased proportionally with nutrient enrichment in P. acuta, but not in P. compressa, revealing contrasting patterns of host–symbiont acclimatization.The two species also exhibited contrasting effects of nutrient enrichment on skeletal growth. Calcification was enhanced at low nutrient enrichment (1 µM NO3‐) in P. acuta, but comparable to the control at higher concentrations, whereas calcification was reduced in P. compressa (30%–35%) above 3 µM NO3‐.Stable isotope analysis revealed species‐specific nitrogen uptake dynamics in the coral–algal symbiosis. The endosymbionts of P. acuta exhibited increased nitrogen uptake (decreased δ15N) and incorporation (19%–31% decrease in C:N ratios) across treatments. In contrast, P. compressa endosymbionts maintained constant δ15N values and low levels of nitrogen incorporation (9%–11% decrease in C:N ratios). The inability of P. acuta to regulate endosymbiont nutrient uptake may indicate an emerging destabilization in the coral–algal symbiosis under nutrient enrichment that could compromise resistance to additional environmental stressors.Our results highlight species‐specific differences in the coral–algal symbiosis, which influence responses to chronic nutrient enrichment. These findings showcase how symbioses can vary among closely related taxa and underscore the importance of considering how life‐history traits modify species response to environmental change. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge (SGD) is an important transporter of solutes and fresh water in coastal systems worldwide. In high island systems with a mixed semidiurnal tidal cycle driving SGD, coastal biogeochemistry is temporally and spatially variable. Past studies have shown that SGD covaries with the local species composition, diversity, and richness of biological communities on a scale of meters. Empirical orthogonal function analyses (EOF)—a method analogous to principal components analysis which finds spatial patterns of variability and their time variation period—were used to define both the spatial and temporal variation in SGD using spatially resolved time series of salinity. The first two EOFs represented variability at the tidal 12‐h period and the daily 24‐h period, respectively, and were responsible for more than 50% of the SGD‐derived salinity variability. We used the first two EOFs to explore spatiotemporally explicit patterns in SGD variability and their relationships with benthic community structure in reef systems. Distance‐based linear models found significant relationships between multivariate community structure and variability in SGD at different periods. Taxa‐specific logistic regressions showed that zoanthids and turf are more likely to be present in areas with high tidally driven SGD variability, while the inverse relationship is true for the invasive rhodophyte Acanthophora spicifera, calcifying macroalgae, the native rhodophyte Pterocladiella sp., the cyanobacteria Lyngbya sp., and the invasive chlorophyte Avrainvillea amadelpha. These results show that benthic communities vary with respect to SGD derived salinity at the scale of hundreds of meters resulting in spatially heterogeneous biotic patches. [ABSTRACT FROM AUTHOR]

Interactions between dissolved organic carbon (DOC) and bacterioplankton were examined during a diatom and Phaeocystis bloom in the Santa Barbara Channel (SBC) over five days following an upwelling event. The SBC was heterogeneous in physical state (recently upwelled vs. more stratified), nutrient concentration, and productivity, encompassing phytoplankton physiological states from a healthy bloom through the onset of silicon stress. DOC accumulated in the upper 10 m over the bloom, with compositional shifts indicated by chromophoric dissolved organic matter (CDOM) parameters. DOC bioavailability and bacterial growth and community composition responses were assessed with dilution batch-culture bioassays. In these experiments, bacterioplankton DOC usage increased over the bloom, with uptake of 1.5-5.3 μmol L−1 over three days, 1.5-5.7 μmol L−1 over one week, and 1.8-10.8 μmol L−1 over 10 weeks. DOC removal was poorly correlated with traditional proxies of bioavailability (chlorophyll a concentration, elemental ratios of dissolved organic matter, and CDOM). However, bacterial growth efficiency (BGE) was strongly related to in situ conditions, with higher BGEs on fresher, late-bloom DOC. After 10 weeks, 1.6-15.7 μmol L−1 of the DOC that accumulated during the bloom remained unutilized in the bioassays, with higher concentrations of persistent DOC in experiments from senescent bloom physiological states, supporting the putative relationship between phytoplankton blooms and seasonal DOC accumulation in the field. These experiments demonstrate that DOC released by the plankton community during a bloom fuels increased short-term and long-term bacterial activity, enhances presumed trophic transfer via increased BGEs, and leads to the accumulation of persistent, potentially exportable, DOC. © 2015 Association for the Sciences of Limnology and Oceanography [ABSTRACT FROM AUTHOR]

Eutrophication remains one of the foremost impacts of industrialization and population expansion on aquatic ecosystems worldwide. Selecting metrics for assessing the manner in which communities and biogeochemical processes respond to nutrient fertilization is an ongoing management challenge critical both for detecting changes and for monitoring recovery of impaired environments. A key limitation to the selection of response variables is the lack of consistent evaluation of metrics under the same conditions in multiple systems across ecoregions. Here we report the results of nutrient‐diffusing agar fertilization experiments conducted simultaneously in 30 streams in two distinct ecoregions of California, USA: the mountainous Sierra Nevada and the coastal chaparral of Santa Barbara county. In each experiment we evaluated algal community shifts across five nutrient delivery rates using multiple response variables at the ecosystem process (respiration and primary production), community (biomass and diversity metrics), functional group (nitrogen fixing, growth form, nutrient adaptation), and taxonomic group (indicator species, genera, families) levels of ecological organization. We used mixed‐effect general linear models to quantify the magnitude, sensitivity, and consistency of responses among streams within and across ecoregions to provide an objective assessment of the potential for each variable to describe and detect significant changes in algal community characteristics. Our results indicate that ecosystem‐ and community‐level variables showed significant and consistent nutrient responses among diverse streams and across the two ecoregions, while indicator taxa and functional groups were less likely to respond consistently to nutrient enrichment. We discuss the relevance of our findings to the ongoing development of monitoring and bioassessment strategies for aquatic eutrophication. [ABSTRACT FROM AUTHOR]

The understanding of biogeochemical cycles has benefited from technological advances facilitating new kinds of measurements and observations. Satellite-borne ocean-color sensors that assess the physiological status of phytoplankton have led to improved estimates of oceanic productivity, as have micrometeorological approaches measuring terrestrial photosynthesis and respiration. The advent of satellites fitted with synthetic aperture radar (a specialized sensor used to determine inundation extent and vegetation types in wetlands) has revealed large fluxes of carbon dioxide and methane from these areas. Advances in the measurement of chemical constituents and turbulence have allowed the detection of high-resolution coupling between physical and biogeochemical processes. Genomics and proteomics - the study of genes and of an organism's complement of proteins, respectively - have revolutionized our understanding of the types of cells present in the environment and their ability to transform elements by allowing direct assessment of gene and protein sequences. [ABSTRACT FROM AUTHOR]

The idea that productivity regulates species diversity is deeply ingrained in the field of ecology. Yet, over the past few decades, an increasing number of experiments have shown that species diversity controls, rather than simply responds to, biomass production. These contrasting perspectives have led to a seeming paradox: Is diversity the cause or the consequence of biological production? Here we present empirical evidence for the multivariate productivity--diversity (MPD) hypothesis, which argues that differing perspectives on productivity--diversity relationships can be resolved by recognizing that historical research has focused on how resource supply regulates both the productivity and richness of local competitors, whereas more recent studies have focused on how the richness of a colonist pool regulates the efficiency by which resources are captured and converted into new tissue. The MPD hypothesis predicts that three pathways operate concurrently to generate productivity--diversity relationships in nature: (1) resource supply directly limits the standing biomass and/or rate of new production by primary producers, (2) producer biomass is directly influenced by the richness of species that locally compete for resources, and (3) resource supply rate indirectly affects producer biomass by influencing the fraction of species from a colonist pool that locally coexist. To examine whether this set of pathways explains covariation between productivity and diversity in natural streams, we used nutrient-diffusing agar "patches" to manipulate resource supply rates in 20 streams throughout the Sierra Nevada mountain range, California, USA. We then measured the fraction of periphyton species from the stream colonist pool co-occurring on each nutrient patch, as well as the standing biomass and rates of primary production. Natural patterns of covariation agreed with predictions of the MPD hypothesis. Algal biomass was an increasing function of nutrient supply, and an increasing function of local richness. The fraction of species from the colonist pool found co- occurring on a patch was a concave-down function of nutrient supply, causing nutrients to indirectly affect biomass via control over local richness. These results suggest that the MPD hypothesis is a viable explanation of patterns of diversity and productivity in natural stream ecosystems, and that it has potential to merge the historical view that productivity drives diversity with a parallel view that diversity drives productivity. [ABSTRACT FROM AUTHOR]

We examined bacterioplankton community composition (BCC; using polymerase chain reaction-based phylogenetic fingerprinting) and the composition of dissolved organic matter (DOM; using scanning fluorometry) within lake chains in high-elevation catchments of the Sierra Nevada, California. We compared temporal BCC dynamics in two connected cirque lakes and their inlets and found that lake : inlet community similarity of both lakes declined with increasing residence time because of seasonal snowpack melting. However, for over 3 yr the headwater lake remained distinct from its inlet even during peak snowmelt (residence time <5 d), whereas the downstream lake was consistently similar to both its inlet and the upstream lake. We subsequently sampled 17 lake chains (4-8 lakes each) throughout the Sierra Nevada longitudinally to examine how landscape position and catchment characteristics influenced the similarity in BCC and DOM among lakes and their connecting streams. Headwater lakes were significantly less similar to their inlet streams than were downstream lakes. BCC was heterogeneous throughout each catchment, but headwater inlet BCC was consistently distinct from all downstream samples. In contrast, DOM exhibited a trend of increasing homogeneity as water moved downstream: upstream and downstream waters were distinct but without clear headwater outliers. The similarity in BCC and DOM between any two lakes within a chain declined with geographic distance. Our results illustrate the contrasting roles of environmental selection and dispersal in structuring bacterioplankton communities, supporting both strong within-lake environmental selection for communities distinct from terrigenous inputs and a weaker effect of geographic dispersal differentiating communities among lakes linked by advective flow. [ABSTRACT FROM AUTHOR]