Genetic distance predicts trait differentiation at the subpopulation but not the individual level in eelgrass, Zostera marina.

Ecological studies often assume that genetically similar individuals will be more similar in phenotypic traits, such that genetic diversity can serve as a proxy for trait diversity. Here, we explicitly test the relationship between genetic relatedness and trait distance using 40 eelgrass (Zostera marina) genotypes from five sites within Bodega Harbor, CA. We measured traits related to nutrient uptake, morphology, biomass and growth, photosynthesis, and chemical deterrents for all genotypes. We used these trait measurements to calculate a multivariate pairwise trait distance for all possible genotype combinations. We then estimated pairwise relatedness from 11 microsatellite markers. We found significant trait variation among genotypes for nearly every measured trait; however, there was no evidence of a significant correlation between pairwise genetic relatedness and multivariate trait distance among individuals. However, at the subpopulation level (sites within a harbor), genetic (FST) and trait differentiation were positively correlated. Our work suggests that pairwise relatedness estimated from neutral marker loci is a poor proxy for trait differentiation between individual genotypes. It remains to be seen whether genomewide measures of genetic differentiation or easily measured "master" traits (like body size) might provide good predictions of overall trait differentiation

Long-term changes in a zooplankton community revealed by the sediment archive

To reconstruct changes in zooplankton communities in response to past anthropogenic perturbations, one possibility is to use the sedimentary records. We analyzed the sediments at a coastal site in the Northern Baltic Sea to relate changes in the zooplankton community to anthropogenic eutrophication and the invasion of a predatory cladoceran, Cercopagis pengoi. We sampled 30-cm laminated sediment cores and dated the sediment layers back to the 1950s. From each 1-cm layer, we measured eutrophication indicators (delta C-13, delta N-15, TC, TN, TP) and identified and counted zooplankton resting eggs (cladoceran, calanoid copepod, rotifer). In addition, we estimated the abundance of the cladoceran Bosmina (Eubosmina) maritima by counting subfossils (carapaces, headshields, and ephippia) and estimated the experienced stress as the relationship between sexual and asexual reproduction. Using redundancy and variance partitioning analyses, we found similar to 16% of the variation in the zooplankton community to be explained by eutrophication, and 24% of the variation in B. (E.) maritima abundance and reproduction mode to be explained by eutrophication and the introduction of the alien predator. Our results show a long-term shift from calanoid copepods and predatory cladocerans toward small-sized zooplankton species, like rotifers. Furthermore, the results indicate that the invasion of C. pengoi induced a short-term increase in sexual reproduction in B. (E.) maritima. The results indicate that anthropogenic eutrophication since the 1950s has altered the zooplankton community toward smaller species, while the invasion of the predatory cladoceran had only a transitory influence on the community during its expansion phase.Peer reviewe

Primary, new and export production in the NW Pacific subarctic gyre during the vertigo K2 experiments

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1594-1604, doi:10.1016/j.dsr2.2008.04.013.This paper presents results on tracer experiments using 13C and 15N to estimate uptake rates of dissolved inorganic carbon (DIC) and nitrogen (DIN). Experiments were carried out at station K2 (47°N, 161°E) in the NW Pacific subarctic gyre during July-August 2005. Our goal was to investigate relationships between new and export production. New production was inferred from the tracer experiments using the f ratio concept (0-50m); while export production was assessed with neutrally buoyant sediment traps (NBSTs) and the e ratio concept (at 150m). During trap deployments, K2 was characterized both by changes in primary production (523 to 404 mg C m-2 d-1), new production (119 to 67 mg C m-2 d-1), export production (68 to 24 mg C m-2 d-1) and phytoplankton composition (high to low proportion of diatoms). The data indicate that 17 to 23% of primary production is exportable to deeper layers (f ratio) but only 6 to 13% collected as a sinking particle flux at 150m (e ratio). Accordingly, > 80% of the carbon fixed by phytoplankton would be mineralized in the upper 50m (1 – f), while < 11% would be within 50-150m (f – e). DIN uptake flux amounted to 0.5 mM m-2 h-1, which was equivalent to about 95% particulate nitrogen (PN) remineralized and/or grazed within the upper 150 m. Most of the shallow PN remineralization occurred just above the depth of the deep chlorophyll maximum (DCM), where a net ammonium production was measured. Below the DCM, while nitrate uptake rates became negligible because of light limitation, ammonium uptake did continue to be significant. The uptake of ammonium by heterotrophic bacteria was estimated to be 14-17% of the DIN assimilation. Less clear are the consequences of this uptake on the phytoplankton community and biogeochemical processes, e.g. new production. It was suggested that competition for ammonium could select for small cells and may force large diatoms to use nitrate. This implies that under Fe stress as observed here, ammonium uptake is preferred and new production progressively suppressed despite the surplus of nitrate.This research was supported by the Research Foundation Flanders through grant G.0021.04 and Vrije Universiteit Brussel via grant GOA 22, as well as the US National Science Foundation programs in Chemical and Biological Oceanography

Positive Feedbacks in Seagrass Ecosystems – Evidence from Large-Scale Empirical Data

Positive feedbacks cause a nonlinear response of ecosystems to environmental change and may even cause bistability. Even though the importance of feedback mechanisms has been demonstrated for many types of ecosystems, their identification and quantification is still difficult. Here, we investigated whether positive feedbacks between seagrasses and light conditions are likely in seagrass ecosystems dominated by the temperate seagrass Zostera marina. We applied a combination of multiple linear regression and structural equation modeling (SEM) on a dataset containing 83 sites scattered across Western Europe. Results confirmed that a positive feedback between sediment conditions, light conditions and seagrass density is likely to exist in seagrass ecosystems. This feedback indicated that seagrasses are able to trap and stabilize suspended sediments, which in turn improves water clarity and seagrass growth conditions. Furthermore, our analyses demonstrated that effects of eutrophication on light conditions, as indicated by surface water total nitrogen, were on average at least as important as sediment conditions. This suggests that in general, eutrophication might be the most important factor controlling seagrasses in sheltered estuaries, while the seagrass-sediment-light feedback is a dominant mechanism in more exposed areas. Our study demonstrates the potentials of SEM to identify and quantify positive feedbacks mechanisms for ecosystems and other complex systems

Retardation of arsenic transport through a Pleistocene aquifer

Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast Asia exposes an estimated population of over a hundred million people to toxic levels of arsenic1. Holocene aquifers are the source of widespread arsenic poisoning across the region2, 3. In contrast, Pleistocene sands deposited in this region more than 12,000 years ago mostly do not host groundwater with high levels of arsenic. Pleistocene aquifers are increasingly used as a safe source of drinking water4 and it is therefore important to understand under what conditions low levels of arsenic can be maintained. Here we reconstruct the initial phase of contamination of a Pleistocene aquifer near Hanoi, Vietnam. We demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a Holocene aquifer into a previously uncontaminated Pleistocene aquifer. We also find that arsenic adsorbs onto the aquifer sands and that there is a 16–20-fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period. Our findings suggest that arsenic contamination of Pleistocene aquifers in south and southeast Asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport.National Science Foundation (U.S.) (NSF grant EAR09-11557)Swiss Agency for Development and Cooperation (Grant NAFOSTED 105-09-59-09 to CETASD, the Centre for Environmental Technology and Sustainable Development (Vietnam))National Institute of Environmental Health Sciences (NIEHS grant P42 ES010349)National Institute of Environmental Health Sciences (NIEHS grant P42 ES016454

Positive Feedbacks in Seagrass Ecosystems – Evidence from Large-Scale Empirical Data

Positive feedbacks cause a nonlinear response of ecosystems to environmental change and may even cause bistability. Even though the importance of feedback mechanisms has been demonstrated for many types of ecosystems, their identification and quantification is still difficult. Here, we investigated whether positive feedbacks between seagrasses and light conditions are likely in seagrass ecosystems dominated by the temperate seagrass Zostera marina. We applied a combination of multiple linear regression and structural equation modeling (SEM) on a dataset containing 83 sites scattered across Western Europe. Results confirmed that a positive feedback between sediment conditions, light conditions and seagrass density is likely to exist in seagrass ecosystems. This feedback indicated that seagrasses are able to trap and stabilize suspended sediments, which in turn improves water clarity and seagrass growth conditions. Furthermore, our analyses demonstrated that effects of eutrophication on light conditions, as indicated by surface water total nitrogen, were on average at least as important as sediment conditions. This suggests that in general, eutrophication might be the most important factor controlling seagrasses in sheltered estuaries, while the seagrass-sediment-light feedback is a dominant mechanism in more exposed areas. Our study demonstrates the potentials of SEM to identify and quantify positive feedbacks mechanisms for ecosystems and other complex systems

Reversed flow of Atlantic deep water during the Last Glacial Maximum

The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies1, 2 and recent model simulations3 indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic 231Pa/230Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength4, 5, 6. Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in 231Pa/230Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of 231Pa/230Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC—with a prominent southerly flow of deep waters originating in the North Atlantic—arose only during the Holocene epoch