bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental paleoecology (resurrection ecology): Chasing Van Valen's Red Queen hypothesis
Authors: Kerfoot W. Charles , Weider Lawrence J.
Published: July 2010

Taking an experimental approach to paleoecology, we evaluated the prey portion of Van Valen's Red Queen hypothesis by retrieving diapausing eggs from lake sediments for laboratory tests of evolutionary responses. The sediments consisted of core samples from Portage Lake (Michigan), which were dated by a combination of varve counts and radioisotope (137Cs and 210Pb) techniques. The sedimentary record of Portage Lake documented several major environmental changes (mining, channeling, post‐1950s eutrophication) over the past century. Remains of Leptodora, Chaoborus, and Polyphemus from sediments and historic fish surveys suggested a changing balance of invertebrate and vertebrate (fish) predators. Daphnia retrocurva was present in Portage Lake for over 80 yr and replaced Daphnia dentifera during the eutrophication period. Against the background of environmental change, we tested a part of the Red Queen hypothesis (i.e., that the prey species D. retrocurva must be continually evolving relative to its primary invertebrate predator, just to remain in place). We examined D. retrocurva morphology and genetics through time by retrieving and hatching diapausing eggs from different sediment strata for genetic characterization and by common garden experiments (i.e., isolates reared under the same environmental conditions). Mitochondrial DNA 12S/16S analyses and allozyme electrophoresis were used to characterize hatchlings from various levels. Allele frequencies at one allozyme (Pgi) locus in D. retrocurva differed over the eutrophication phase, suggesting founder effects, natural selection, or both. Common garden experiments with D. retrocurva hatchlings documented significant microevolutionary adjustments in both helmet and spine lengths, supporting the hypothesis of continual evolution in prey against a changing balance of predation.
Rotifers from diapausing, fertilized eggs: Unique features and emergence
Authors: Gilbert John J. , Schröder Thomas
Published: July 2010

Sexual reproduction in cyclically parthenogenetic rotifers results in the production of diapausing, fertilized (resting) eggs, which can survive for decades in sediment egg banks. Stem females hatching from these eggs can differ from genetically identical ones produced in subsequent parthenogenetic generations. Differences involve the inhibition of sexual reproduction, which may persist for many parthenogenetic generations; reduced morphology (apterous Polyarthra, shorter‐spined Brachionus); and larger stores of lipid. These features of stem females may facilitate colonization by favoring population growth via female parthenogenesis and by decreasing food requirements for survival and reproduction. Fertilized eggs in dried sediments probably hatch soon after being flooded by rising water levels. Eggs in permanently submerged sediments may be in environments that inhibit hatching (e.g., darkness, low water temperature, low dissolved oxygen) and hatch only when brought to the sediment surface or resuspended into the water column by currents or bioturbation. Laboratory experiments show that the hatching of diapausing, fertilized eggs can require light and temperatures suitable for population development or specific changes in temperature consistent with a new growing season. The limited data that exist on their emergence from fertilized eggs in natural systems come from sediment traps, from the occurrence in the water column of apterous Polyarthra or empty egg shells, and from calculations of negative mortality rates of planktonic populations. Fertilized eggs often hatch at the beginning of the growth season but may continue to hatch intermittently thereafter. The extremely high genotypic diversity of some natural rotifer populations indicates extensive hatching of stem females from the sediment egg bank.
Zooplankton recovery after fish removal: Limitations of the egg bank
Authors: Sarnelle Orlando , Knapp Roland A.
Published: July 2010

The Allee effect, operating via mate limitation, theoretically reduces the probability of recovery from local extinction of dioecious zooplankton relative to species that can reproduce asexually. We removed fish from four alpine lakes in the Sierra Nevada in which both Hesperodiaptomus shoshone (a calanoid copepod) and Daphnia middendorffiana were historically present but had become locally extinct after fish stocking. After complete fish removal, D. middendorffiana returned to all four lakes, whereas H. shoshone failed to return in any of the lakes, despite diapausing eggs observed in the sediments of two lakes and a few individuals in the water column of one lake during the first summer after fish removal. We estimated the potential magnitude of the Allee effect, as it may have affected the recovery of H. shoshone, by comparing estimates of minimum founding population size of H. shoshone with estimates of actual founding population sizes of D. middendorffiana. It took 4 yr for D. middendorffiana to recover to detectable levels in one of the four lakes, which we suggest was the result of a very small founding population. The latter was three to four orders of magnitude smaller than the minimum founding population size we calculated for H. shoshone, indicating the potential for a large effect of mate limitation on the copepodÂs ability to recover. H. shoshone might never return to these lakes without human intervention because of the combined effects of mate limitation and low rates of overland dispersal.
Do parasites lower Daphnia hybrid fitness?
Authors: Wolinska Justyna , Keller Barbara , Bittner Kerstin , Lass Sandra , Spaak Piet
Published: July 2010

In Greifensee (Switzerland), Daphnia galeata × hyalina hybrids cooccur with both parental taxa. Hybrids are the most abundant taxon, suggesting that hybrids have greater fitness. In addition to many known factors that promote hybrids, specific environmental conditions favoring parentals must also exist to explain their cooccurrence. We investigated the influence of the protozoan gut parasite Caullerya mesnili on the D. galeata × hyalina species complex. Up to 22% of the Daphnia population was infected with C. mesnili in October 2002. C. mesnili dramatically reduced the fecundity of its hosts. Only 2% of infected individuals carried eggs compared with 70% in the uninfected group, which suggests that C. mesnili exerts a strong selection pressure. Our results indicate that hybrids were frequently infected, whereas parental D. galeata were almost never infected. We also found genetic variation for infection within hybrids, evidenced by significant differences in clonal composition between the infected and uninfected parts of the taxon. Resistance of D. galeata might counterbalance the greater fitness of hybrids and therefore contribute to the maintenance of coexistence of hybrids and parentals in this lake.
Associations among ribosomal (r)DNA intergenic spacer length, growth rate, and C:N:P stoichiometry in the genus Daphnia
Authors: Weider Lawrence J. , Glenn Karen L. , Kyle Marcia , Elser James J.
Published: July 2010

The growth rate hypothesis (GRH) proposes that variation in organismal C:N: P ratios is associated with growth rate due to differential allocation to P‐rich ribosomal RNA to support growth. A further extension of the GRH proposes that this variation is associated with the structure of ribosomal (r)DNA genes (e.g., promoter‐enhancer sequences in the rDNA intergenic spacer [IGS]) that influence the production rate of rRNA. To test these ideas, we examined growth rate, rDNA structure, RNA:DNA ratios, and elemental composition (C :N: P) in multiple clones of three Daphnia species (pulex, pulicaria, magna) in controlled laboratory experiments. Results show that a portion of the rDNA motif, consisting primarily of the IGS, shows considerable interspecific and intraspecific length variation. Significant interspecific differences in all parameters were found, while significant intraspecific (clone) effects were detected only for RNA:DNA ratios and growth rate. As predicted by the GRH, there were significant positive correlations between RNA:DNA ratio and either growth rate or rDNA/IGS length, and a significant positive correlation between rDNA/IGS length and growth rate, when clonal means for all three species were examined. However, no clear‐cut relationships between RNA:DNA and either C: P or N: P were observed for any of the three species. Likewise, no discernible intraspecific relationships between growth rate and either C: P or N: P were detected. These preliminary findings suggest that further study of elemental composition, nucleic acid content, and growth rates of organisms may lead to a clearer understanding of the role of stoichiometric constraints in influencing the ecology and evolution of natural populations.
Effects of microzooplankton and mixotrophy in an experimental planktonic food web
Authors: Ptacnik Robert , Sommer Ulrich , Hansen Thomas , Martens Volker
Published: July 2010

Microzooplankton have received increased attention as an important trophic link between the microbial loop and calanoid copepods. On the basis of food size spectra overlap in some microzooplankton groups and calanoid copepods, however, such microzooplankton could function as competitors rather than as food for calanoid copepods (intraguild prey). Mixotrophic flagellates presumably represent a link between the microbial loop and the micro and mesozooplankton. We investigated the effects of microzooplankton and mixotrophy by altering the presence of a heterotrophic dinoflagellate and of a mixotrophic nanoflagellate in artificial food webs with calanoid copepods as terminal consumers. Overall system productivity was manipulated by two levels of nutrient enrichment. The heterotrophic dinoflagellate drastically reduced the nanophytoplankton and enhanced the reproduction of the copepods, suggesting that its role as a competitor is negligible compared to its function as a trophic link. In spite of the presence of heterotrophic nanoflagellates, the mixotroph had a strong negative effect on the picophytoplankton and (presumably) on bacterial biomass. At the same time, the mixotroph enhanced the atomic C:N ratio of the seston biomass, indicating a higher efficiency in overall primary production. Copepod reproduction was enhanced in the presence of the mixotrophic nanoflagellate. Results did not support predictions of the intraguild predation theory: The ratios of the intraguild predators and their preys were not affected by overall system productivity.
Fingerprints of biocomplexity: Taxon‐specific growth of phytoplankton in relation to environmental factors
Authors: Lehman John T. , Abella Sally E. B. , Litt Arni H. , Edmondson W. Thomas
Published: July 2010

Phytoplankton and environmental conditions in Lake Washington, Seattle, Washington, are discussed from the perspective of dynamic relationships between taxon‐specific growth rates and environmental variables. More than four decades of measurements permit inspection of conditions associated with net increase and decrease for 40 phytoplankton species or species groups. Reproducible patterns exist for growth responses to over 25 environmental factors including nutrient chemistry, physical variables, and herbivorous zooplankton species. There appear to be no more than six main modalities of response to environmental factors, and responses to chemical and physical variables show coherence across taxa. Diatoms show a near uniform positive growth response to abundant inorganic nutrients, cold and transparent water, deep mixing, and intolerance for virtually all zooplankton grazers. Many chlorophytes and cyanobacteria show equally uniform growth responses to chemical and physical variables, although their preferences are virtually opposite from the diatoms. They benefit from the presence of copepods but show highly specific growth rate responses to different cladocerans and rotifers. Growth rate variations among the diatoms sort out along gradients of resource and physical factors, but there is coherence to the rise and fall of multiple species. Among the other algal divisions, despite a common set of physical and chemical conditions that promote growth rates, the species do not increase and decrease together. Instead, the prevailing grazer community appears to shape the phytoplankton community by admitting only certain species from the large pool of contenders.
Distribution and retention of effluent nitrogen in surface sediments of a coastal bay
Authors: Savage Candida , Leavitt Peter R. , Elmgren Ragnar
Published: July 2010

Anthropogenic nitrogen (N) often causes coastal eutrophication, yet little is known about the fate and retention of effluent N in coastal waters and, hence, about the system’s ability to assimilate excess N loads. We used the spatial distribution of stable N isotope ratios and algal pigments in sedimentary organic matter from a Baltic bay receiving tertiary‐treated effluent to evaluate the extent of effects and the role of nearshore marine environments as sinks of anthropogenic N. Surface sediments (0–2 cm and 2–4 cm) exhibited a pronounced spatial gradient of δ15N, with the most elevated values (~8‰) near the outfall; values decreased linearly to values of ;4‰ outside the bay. Sedimentary pigment concentrations were consistent with water‐column data and showed that phytoplankton biomass was elevated in the inner reaches of the bay. In particular, diatoms were heavily labeled (δ15N ~ 10‰), reached maximum abundance near the effluent outfall, and were likely the main mechanism delivering effluent N to the sediments. Sediments within the bay removed ~5–11% of wastewater N inputs, with 50% of the sequestered effluent N buried in the basin nearest to the outfall. Magnitudes of N removal by sediments (23–26 x 104 kg N yr−1) were less than those estimated for denitrification (30–60 x 104 kg N yr−1), but they were substantially greater than biological uptake by macroalgae (~2 x 104 kg N yr−1). Taken together, these patterns demonstrate the idea that coastal sediments can be effective sinks of wastewater N, even after 30 yr of effluent input.
Quantifying habitat‐specific diatom production: A critical assessment using morphological and biogeochemical markers in Antarctic marine and lake sediments
Authors: Verleyen Elie , Hodgson Dominic A. , Leavitt Peter R. , Sabbe Koen , Vyverman Wim
Published: July 2010

Reconstructions of historical primary production, and of the algal groups and habitats that contribute to it, are fundamental in studies of climate and environmental change in both marine and freshwater environments. The aims of this study were to critically evaluate morphological and biogeochemical markers of diatom production by direct comparison of diatom marker pigments with absolute diatom biovolume and to partition diatom production between the main habitats (plankton, sea ice, and benthos). Sediments in two cores from the Larsemann Hills, Antarctica, spanning the last 10,000 yr, were analyzed for siliceous microfossils by microscopy and for fossil pigments by high‐performance liquid chromatography. Diatom pigments (diadinoxanthin, diatoxanthin, fucoxanthin) were highly correlated (r2 = 0.557 and 0.358, p < 0.0001) with diatom biovolume in the marine intervals of both cores, but only weakly correlated in the lacustrine sections (r2 = 0.102, p = 0.111; r2 = 0.223, p = 0.001, after correction for temporal autocorrelation), possibly because of frustule dissolution and selective degradation of diadinoxanthin and diatoxanthin. In contrast, fucoxanthin was better preserved. By combining both microfossil and pigment proxies, we obtained a first estimate of diatom production in specific habitats (benthic and planktonic). Benthic diatom production was greatest in the lacustrine core sections, when benthic microbial mats dominated the flora, whereas diatoms were associated mainly with the water column and sea ice during the marine intervals. The combination of both proxies in marine and freshwater environments permits more accurate interpretation of pigment and diatom data in paleo‐ and neoecological research and the partitioning of diatom production between habitats.
Significance and mechanisms of photosynthetic production of dissolved organic carbonin a coastal eutrophic ecosystem
Authors: Marañón Emilio , Cermeño Pedro , Fernández Emilio , Rodríguez Jaime , Zabala Laura
Published: July 2010

We have determined the seasonal (July 2001‐July 2002) and vertical variability in the photosynthetic production of dissolved organic carbon (DOCp) and particulate organic carbon (POCp) in a coastal upwelling ecosystem (Ría de Vigo, Northwest Spain), together with the relationship between irradiance and DOCp and the time‐course of DOCp over 24‐h periods. Euphotic layer‐integrated rates of DOCp and POCp ranged between 5 and 190 mg C m−2 h−1 and between 40 and 1,130 mg C m−2 h−1, respectively. Irradiance was the most important variable affecting the vertical variability of the percentage of extracellular release [PER, defined as DOCp/(DOCp + POCp)]. Whereas POCp decreased markedly below the surface, DOCp remained constant or even increased, thus causing a sharp increase in PER with depth. Biomass‐specific rates of DOC production also increased with depth. These observations were confirmed by the results of photosynthesis‐irradiance experiments, which consistently showed highest DOCp and PER values at subsaturating irradiances. Our results argue against the view that the release of DOC is an overflow mechanism occurring preferentially under conditions of high irradiance and low nutrient concentration. PER was uncorrelated with the size structure of phytoplankton biomass and productivity, and >80% of the variability in integrated DOCp was explained by POCp. These findings indicate that the relative importance of dissolved primary production was independent of the dominant type of planktonic trophic organization. Moreover, production of DOC stopped at night, which strongly indicates that trophic processes were not involved in the release of dissolved photosynthate. Our data support a purely physiological mechanism of passive DOC release by normally growing cells, which is enhanced under suboptimal irradiances but proceeds at a similar biomass‐specific rate throughout the year. On an integrated basis, PER averaged 19 ± 1%, thus indicating that even in eutrophic waters, total primary productivity can be significantly underestimated if the dissolved products of photosynthesis are not taken into account.
Primary consumers and particulate organic matter: Isotopic evidence of strongselectivity in the estuarine transition zone
Authors: Martineau Christine , Vincent Warwick F. , Frenette Jean‐Jacques , Dodson Julian J.
Published: July 2010

The freshwater−saltwater transition zones of upper estuaries often contain high concentrations of particulate organic matter (POM) that could potentially support their productive food webs. Our objectives were to define the carbon and nitrogen isotopic characteristics of POM across the estuarine transition zone (ETZ) of the St. Lawrence River, Canada, and to determine the availability of this material to primary consumers. The δ13C of seston from upstream freshwater samples (−26.3 ± 0.9‰) indicated a large contribution of terrestrially derived organic material, whereas downstream sites had higher values (−24.8 ± 0.5‰) attributable to dilution with marine phytoplankton. The ETZ turbidity maximum had more negative values (−27.0 ± 1.6‰), especially the <5−mm fraction (−30.0 ± 1.3‰) that contributed 56−86% of the POM. The δ13C of the dominant consumer species in the ETZ (Bosmina longirostris, Keratella sp., Eurytemora affinis, and adult Dreissena polymorpha) averaged −21.0 ± 0.9‰, well above the seston values. The δ15N of the three animals of lowest trophic position averaged 7.3 ± 0.8‰, less than 2‰ above the seston δ15N (5.7 ± 0.2‰ for all sites). Our results imply that the bulk POM is largely detrital and unavailable to the ETZ food web and that the primary consumers feed selectively on phytoplankton despite its low contribution (<10%) to total POM. A cross−system comparison of isotopic data shows that the large enrichment of 13C in consumers relative to POM is unusual, underscoring the distinctive character of turbid, upper estuarine ecosystems.
Neither with nor without you: A complex algal control on bacterioplankton in a high mountain lake
Authors: Medina‐Sánchez Juan Manuel , Villar‐Argaiz Manuel , Carrillo Presentación
Published: July 2010

The literature proposes that the microbial loop is a key link for ecosystem functioning, particularly in oligotrophic conditions. From original and published data for a period spanning 1986 to 1999, we examined the food web structure of a clear‐water, oligotrophic, high mountain lake. The heterotrophic microbial food web was weakly developed in a grazing food chain dominated by copepods and phytoplankton, the latter mainly composed of mixotrophic flagellates. Bacteria constituted a minor component of the plankton community in terms of abundance, biomass, and production, in contrast to the situation usually reported in oligotrophic conditions. Abiotic and biotic factors that regulate bacterioplankton biomass and production were assessed, including the bacterivory capability of mixotrophic flagellates experimentally quantified by using 3H‐thymidine as a tracer. Algae were the main factor controlling bacterioplankton. Their regulatory effect has a dual nature: (1) a resource‐based control, through the dependence of the bacteria on photosynthetic carbon released by algae, i.e., a commensalistic interaction (“without you I cannot live”) and (2) a predatory control, with bacteria as prey for mixotrophs (“with you I die”). Mixotrophic metabolism can constitute an adaptive strategy for algae to overcome ultraviolet (UV) stress, by using bacteria as a source of carbon and mineral nutrients in conditions of inhibited photosynthesis and mineral nutrient uptake. Mixotrophy acts as a bypass of carbon flux toward the grazing food chain, explaining the scarce development of the heterotrophic microbes in this and other high mountain lakes.
Viral release of iron and its bioavailability to marine plankton
Authors: Poorvin Leo , Rinta‐Kanto Johanna M. , Hutchins David A. , Wilhelm Steven W.
Published: July 2010

The biological availability of Fe has been demonstrated to strongly influence both primary and secondary production in pelagic as well as coastal upwelling high‐nutrient low‐chlorophyll (HNLC) regimes. Although nearly all of the dissolved Fe in marine surface waters is thought to be complexed by organic ligands, the character and origin of these Fe‐organic complexes remains a mystery. Here we report that the activity of naturally occurring viral populations in an HNLC coastal upwelling system can regenerate sufficient concentrations of dissolved Fe to support the growth of the native phytoplankton community. When combined with studies that have demonstrated that Fe in virus‐mediated lysates of heterotrophic bacteria and cyanobacteria is highly bioavailable to model marine plankton, our data demonstrate that viral activity in this marine system (and potentially others) is critical to the recycling of organically complexed Fe that supports as much as 90% of primary production in HNLC surface waters.
Novel copper‐binding and nitrogen‐rich thiols produced and exuded by Emiliania huxleyi
Authors: Dupont Christopher L. , Nelson Robert K. , Bashir Saj , Moffett James W. , Ahner Beth A.
Published: July 2010

In surface seawater, dissolved copper (Cu) is predominantly bound by a small pool of strong ligands, with further complexation by a larger pool of weaker ligands. Although the chemical structure of both classes of ligands observed in the open ocean remains unknown, it is believed that the stronger ligands are of biological origin, and it has been hypothesized that they are thiols. Using precolumn derivatization high‐performance liquid chromatography electrospray ionization ion‐trap mass spectrometry, we found that the ubiquitous coccolithophorid Emiliania huxleyi constitutively produces two previously uncharacterized thiols, arginine‐cysteine and glutamine‐cysteine, in high intracellular concentrations. E. huxleyi exudes these novel thiols, along with cysteine, in response to increased Cu concentrations in the growth media. Furthermore, stable Cu (I) complexes with the exuded thiols were observed in the growth media using matrix‐assisted laser desorption time‐of‐flight mass spectrometry following size exclusion chromatography. Additionally, E. huxleyi appears to utilize these novel thiols in nitrogen storage and assimilation, as they are rapidly synthesized upon nitrogen addition to a nitrogen‐depleted culture. The identification of specific ligands produced by this organism will facilitate further linkages between this likely source and the pools of Cu‐binding ligands observed in surface seawater, while the discovery of novel thiols signifies the presence of unique and previously unknown biochemical pathways in E. huxleyi.
Nutrient and growth dynamics of Halimeda tuna on Conch Reef, Florida Keys: Possible influence of internal tides on nutrient status and physiology
Authors: Smith Jennifer E. , Smith Celia M. , Vroom Peter S. , Beach Kevin L. , Miller Steven
Published: July 2010

We conducted a manipulative nutrient enrichment study to examine the physiological and growth dynamics of the common tropical reef alga Halimeda tuna from shallow and deep coral reef environments on Conch Reef, in the Florida Keys. Paradoxically, H. tuna exhibited higher growth rates at depth (low light and below saturating irradiances) than at shallow sites (high light and at or above saturating irradiances). We hypothesized that the differences in growth rates were caused by differing nutrient environments at the two sites potentially caused by the influence of internal tidal bores that elevate nutrient concentrations with depth on Conch Reef. We tested this hypothesis by manipulating nutrients in a 10‐d field experiment, after which we assessed growth, photosynthetic pigments, tissue nutrients, and other physiological parameters. H. tuna from the shallow back reef site exhibited nutrient limitation, as indicated by increases in growth rates, pigmentation, tissue nutrients, segment size, and photosynthetic rates, after enrichment. At the deep site, growth rates were not significantly different between controls and nutrient‐enriched algae. Shallow enriched samples achieved levels of growth that were not significantly different from deep control or enriched samples. Algae from the deep site responded positively to enrichment for some physiological parameters; this suggests an opportunistic strategy in an environment that is known to experience frequent and significant pulses of nutrients from internal tides. Our results document differential nutrient limitation for H. tuna from two sites on Conch Reef where, in general, algae from the shallow site were more nutrient limited than those from the deep site. Finally, this provides some evidence that tropical reef communities may be adapted to large‐scale physical processes such as internal tides.
Direct evidence of a biophysical retention mechanism for coral reef fish larvae
Authors: Paris Claire B. , Cowen Robert K.
Published: July 2010

We examine the hypothesis that reef fish larvae have some direct influence on their own dispersal and ability to recruit to their natal reef by tracking cohorts of bicolor damselfish (Stegastes partitus) from hatching to settlement onto the reef, about 30 d later. We conducted high‐resolution sampling during two consecutive years in a small area (15 km × 20 km) off the west coast of Barbados, extending from depths of 0 to 100 m. Observations of discrete stage‐specific larval patches of mean size of 29.4 and 13.2 km2 for preflexion (1–5‐d old) and flexion/postflexion (.5‐d old) stages extending ca. 30 m in the vertical indicated that larvae initially dispersing as patches tend to stay in coherent patches throughout their pelagic duration. Highest concentrations of preflexion larvae within a patch were in the upper 20 m, while those of older larvae were always deeper. Downward migration of about 60 m throughout ontogeny within stratified currents represented a retention mechanism for locally spawned larvae. Most of the variability in estimated retention rates between daily cohorts occurred during the earliest stages as a result of the dynamic nature of surface currents experienced by larvae prior to the onset of vertical migration. Differences in residence time between experiments were consistent with observed intermonthly variability in recruitment strength, implying that pelagic processes can explain recruitment rates. These results provide empirical evidence for larval retention of coral reef fishes and stress the role of active behavior in larval transport.
New organic matter degradation proxies: Valid in lake systems?
Authors: Meckler A. N. , Schubert C. J. , Cowie G. L. , Peiffer S. , Dittrich M.
Published: July 2010

We determined concentrations and diagenetic state of organic matter in two cores from Lake Zug, Switzerland. To obtain a general picture of organic matter decay in the lake, we measured bulk parameters such as total organic carbon, total nitrogen, and total hydrolyzable amino acids. In addition, we studied specific compound classes in detail—namely, amino acids and chlorins. From these data we obtained two recently proposed indicators for organic matter degradation state—the amino acid‐based degradation index (DI) and the chlorin index (CI). The two indicators complement each other, with the CI being very sensitive during the initial stages of organic matter decay and the DI recording later stages of decay. Further differences are likely caused by nonlinear behavior during degradation. Nonetheless, the two proxies agree in the general picture of the degradation state. Despite higher concentrations of organic matter in the surface sediment at the deeper site, neither of the degradation state indicators showed much difference between the two sites. At depth, the degradation state at the two sites diverges, despite similar concentrations of organic matter. Although we cannot explain all differences between bulk parameters, amino acid composition, and chlorins conclusively, the results indicate a combined effect of different organic matter inputs (e.g., through sediment focusing) and different oxygen availability at the two sites since the onset of eutrophication.
Virus and bacteria dynamics of a coastal sediment: Implication for benthic carbon cycling
Authors: Glud Ronnie N. , Mathias Middelboe
Published: July 2010

We measured microbial heterotrophic activity, bacteria, and virus‐like particle (VLP) abundance in homogenized, undiluted, and anoxic enclosures of sediment collected at a coastal station. The bacterial growth rate and VLP net production increased along with the respiratory activity in response to temperature. This suggests that VLPs represent a dynamic component of benthic microbial communities and that the net production of viriobenthos is regulated by the metabolic activity of bacteria. The abundance, net production, and decay rate of VLPs were significantly higher than those encountered in most pelagic systems. However, the rates were lower than the very few available potential rates (three studies) of viriobenthic activity, which all were obtained applying different slurry approaches. Our measurements support the general observation that virus abundance and production correlate with the trophic status of the environment and show that microbial activity can regulate the viriobenthic production in undiluted, homogenized marine sediments. The virus‐induced bacterial mortality corresponded to ~20% of bacterial net production and ~2% h−1 of the total bacterial population. This is moderate compared with the results of most pelagic studies, and the associated leakage of lysates (dissolved organic carbon) only amounted to 4‐8% of the produced dissolved inorganic carbon. Despite high standing stocks and relatively high turnover rates, VLP‐induced bacterial lysis represented only a minor shunt in the benthic carbon cycle at the investigated site.
Responses of seagrass communities to fertilization along a gradient of relative availability of nitrogen and phosphorus in a carbonate environment
Authors: Ferdie Meredith , Fourqurean James W.
Published: July 2010

Patterns of relative nutrient availability in south Florida suggest spatial differences regarding the importance of nitrogen (N) and phosphorus (P) to benthic primary producers. We did a 14‐month in situ fertilization experiment to test predictions of N and P limitation in the subtropical nearshore marine waters of the upper Florida Keys. Six sites were divided into two groups (nearshore, offshore) representing the endpoints of an N: P stoichiometric gradient. Twenty‐four plots were established at each site with six replicates of each treatment (+N, +P, +N+P, control), for a total of 144 experimental plots. The responses of benthic communities to N and P enrichment varied appreciably between nearshore and offshore habitats. Offshore seagrass beds were strongly limited by nitrogen, and nearshore beds were affected by nitrogen and phosphorus. Nutrient addition at offshore sites increased the length and aboveground standing crop of the two seagrasses, Thalassia testudinum and Syringodium filiforme, and growth rates of T. testudinum. Nutrient addition at nearshore sites increased the relative abundance of macroalgae, epiphytes, and sediment microalgae. N limitation of seagrass in this carbonate system was clearly demonstrated. However, added phosphorus was retained in the system more effectively than N, suggesting that phosphorus might have important long‐term effects on these benthic communities. The observed species‐specific responses to nutrient enrichment underscores the need to monitor all primary producers when addressing questions of nutrient limitation and eutrophication in seagrass communities.
Controls on planktonic metabolism in the Bay of Blanes, northwestern Mediterranean littoral
Authors: Duarte Carlos M. , Agustí Susana , Vaqué Dolors
Published: July 2010

Long‐term and experimental approaches were used to examine the metabolic balance of the planktonic community in the Bay of Blanes (Spanish Mediterranean). Incubation measurements at weekly intervals for 6 yr revealed that community respiration, R, was consistently larger than gross primary production (GPP) by a factor of 2. The plankton community was net heterotrophic for 2/3 of the study period, with a median P/R ratio of 0.65. The biomass of autotrophs comprised, on average, 41% ± 3% of the planktonic biomass, and the total microplankton biomass was about 2.5‐fold greater than that of the primary producers. The monthly average GPP and R were positively correlated with day length, and the planktonic respiration and gross production per unit microplankton biomass increased with increasing water temperature. Experimental nutrient additions had a greater effect on GPP than respiration rates; the increase in R along the nutrient gradient was 7.8% (± 0.4%) of the increase in GPP. As a result, net community production increased in parallel with GPP, shifting from net heterotrophic at low GPP to net autotrophic when GPP increased because of nutrient additions. Our results show that the R that would be supported by llocthonous inputs was 3.83 ± 0.67 µmol O2 L−1 d−1, and the average GPP required to shift the community from net heterotrophic to net autotrophic was about 4 µmol O2 L−1 d−1. This is well above the average GPP recorded in the Bay of Blanes along this study (2.56 ± 0.13 µmol O2 L−1 d−1), explaining the net heterotrophic nature of the community.
Effect of seasonal sediment storage in the lower Mississippi River on the flux of reactive particulate phosphorus to the Gulf of Mexico
Authors: Sutula Martha , Bianchi Thomas S. , McKee Brent A.
Published: July 2010

The annual surface water flux of total reactive (i.e., potentially bioavailable) particulate P from the Mississippi River was estimated by measuring the reactive (including labile, iron, organic, and calcium bound) and nonreactive (detrital) P phases in suspended particulates in the Mississippi River. In addition, the transformation of the major sediment P phases resulting from seasonal channel storage and resuspension was examined. Samples were collected during five cruises over 1 yr at marine and riverine sites. Solid‐phase and pore‐water nutrients were quantified, and solid‐phase P pools were measured using a sequential extraction technique. These results indicate that the Mississippi River exports 134 × 106 kg yr−1 of total reactive P via surface water. Seasonal hydrological forcing controlled the variability in major P phases found in channel sediments through hydrodynamic sorting. Although the 6–9‐month time period during which sediments were stored in the river channel was sufficient to see evidence of early diagenesis in the pore waters, no significant net effect was seen on major P phase distribution. The loss of a significant percentage of labile and iron‐bound P appears to be occurring only as these riverine sediments are deposited and reworked on the continental shelf.
Patterns in microphytobenthic primary productivity: Species‐specific variation inmigratory rhythms and photosynthetic efficiency in mixed‐species biofilms
Authors: Underwood G. J. C. , Perkins R. G. , Consalvey M. C. , Hanlon A. R. M. , Oxborough K. , Baker N. R. , Paterson D. M.
Published: July 2010

The importance of temporal changes in the vertical distribution of microphytobenthic algae on the overall functioning of intertidal biofilms were investigated with low‐temperature scanning electron microscopy and high‐resolution single‐cell fluorescence imaging of photosystem II efficiency (estimated by the fluorescence parameter F’q/F’m) in intact cores maintained in tidal mesocosms. Early morning biofilms consisted of smaller naviculoid and nitzschioid taxa or euglenoid species. By midday, Gyrosigma balticum and Pleurosigma angulatum were dominant. Some taxa (e.g., Plagiotropis vitrea) disappeared from surface layers after midday. Species composition continued to change toward the end of the photoperiod, with G. balticum dominating in diatom‐rich biofilms. In Euglena‐rich biofilms, initial dense surface films of euglenids became progressively dominated by smaller diatoms. F’q/F’m (measured at a photosynthetically active photon flux density (PPFD) of 220 µmol m−2 s−1) of individual cells of all taxa declined significantly after midday, but increased toward dusk. There were significant differences in F’q/F’m between species, particularly after midday. F’q/F’m versus irradiance curves and relative electron transport rate (rETRmax) showed higher efficiencies and rETRmax for euglenids, whereas G. balticum, Nitzschia dubia, and small Nitzschia sp. were shade‐adapted with low values of F’q/F’m, rETRmax, and Esat. G. balticum, P. vitrea, and N. dubia showed rapid vertical migration away from the surface with increasing irradiance. Euglenids, P. angulatum, and N. dubia exhibited their highest rETRmax values at midday. Esat for algal cells was between 500 and 600 µmol m−2 s−1, except for N. dubia and small Nitzschia sp., which had an Esat of 300 µmol m−2 s−1. Differences in behavioral and photophysiological traits between microphytobenthic taxa could be a form of niche separation and need to be incorporated into conceptual models of daily patterns of production in intertidal biofilms.
Long‐term changes in apparent uptake of silica in the San Francisco estuary
Authors: Kimmerer Wim
Published: July 2010

Silica distributions in estuaries can be used to infer uptake, particularly that due to production by diatoms. I used data from two long‐term monitoring programs to infer long‐term patterns of diatom production during summer in the river‐dominated northern San Francisco estuary. This region was most heavily affected by grazing by the clam Potamocorbula amurensis, introduced in 1986. Production based on dissolved silica (Dsi) uptake was high during the 1970s and early 1980s except during a drought in 1976—1977. From 1987 to 2002 production averaged 1% and 17% of the preclam values from the two monitoring programs. Low values in 1976—1977 and after 1986 are consistent with a loss of diatom production due to benthic grazing. There is no evidence of a relation between DSibased production and either freshwater flow or temperature, and therefore climate change is an unlikely influence on the long‐term trend in diatom production in this part of the estuary. Primary production calculated from chlorophyll and extinction coefficient was of a similar magnitude to that calculated from DSi depletion, indicating that most production was due to diatoms during summers of the preclam period.
Siliceous sponges as a silicon sink: An overlooked aspect of benthopelagic coupling in the marine silicon cycle
Authors: Maldonado Manuel , Carmona Ma Carmen , Velásquez Zoila , Puig Angels , Cruzado Antonio , López Angel , Young Craig M.
Published: July 2010

Our current understanding of the silicon (Si) cycle in the ocean assumes that diatoms dominate not only the uptake of silicic acid, but also the production and recycling of biogenic silica (BSi), and that other organisms with siliceous skeletons, including sponges, radiolarians, and silicoflagellates, play a negligible role. In this study, we reexamine some aspects of the potential contribution by sponges and present in vitro evidence that BSi in the form of sponge spicules redissolves into silicic acid at far slower rates than those known for diatom frustules. We also show that the retention of Si by siliceous sponges in some sublittoral and bathyal environments is substantial and that sponge populations function as Si sinks. Additionally, by reanalyzing published information on sponge growth and BSi content, we estimate that BSi production rates by sublittoral sponges in Si‐poor and Si‐rich marine areas fall quite close to values known for diatom assemblages. Therefore, sponges may affect Si cycling dynamics and Si availability for diatoms, particularly in Si‐depleted environments. Altogether, our data strongly suggest that the role of sponges in the benthopelagic coupling of the Si cycle is significant.
The effect of growth rate, phosphorus concentration, and temperature on N2 fixation, carbon fixation, and nitrogen release in continuous cultures of Trichodesmium IMS101
Authors: Mulholland Margaret R. , Bernhardt Peter W.
Published: July 2010

With the use of continuous culture systems, rates of dinitrogen (N2) and carbon (C) fixation and nitrogen (N)‐ and C‐based doubling times were assessed in Trichodesmium IMS101 growing exponentially at steady state dilution rates of 0.10, 0.20, and 0.33 d−1 (doubling times of 10, 5, and 3 d—within the range reported for natural populations). Rates of C fixation, N2 fixation, and N release were examined in replicate culture systems with several techniques. Biomass‐specific C uptake varied little with population doubling time, but N2 fixation and N release varied markedly among treatments. Total daily gross N2 fixation rates and estimated N release rates were higher in cultures with higher dilution rates. Cultures grown at lower dilution rates had higher daily C:N2 fixation ratios and lower N release rates. Consistent with other studies, it was estimated that Trichodesmium released about 80–90% of their recently fixed N2 during growth. Turnover of cellular C estimated from carbon fixation was a good estimator of population growth rates in steady state cultures, whereas turnover of cellular N estimated from gross or net N2 fixation was not. Small changes in temperature (24°C vs. 28°C) did not appear to affect gross N2 fixation, whereas inorganic phosphorus (1 vs. 5 µmol L−1) supply had a large effect on N2 fixation. These results suggest that continuous culture systems are excellent for elucidating physiological responses of Trichodesmium under ecologically relevant growth conditions and provide a framework for assessing highly variable field estimates of N2 and C fixation.
Influence of zooplankton stoichiometry on nutrient sedimentation in a lake system
Authors: Darchambeau François , Thys Isabelle , Leporcq Bruno , Hoffmann Lucien , Descy Jean‐Pierre
Published: July 2010

We explored rates and stoichiometry (C:N: P ratios) of sinking particles in a temperate reservoir during a 2‐yr period. Plankton was sampled weekly, and a sediment trap placed below the metalimnion collected sinking particles. There were no significant relationships between the stoichiometry of entrapped material and seston or zooplankton stoichiometry. However the differences in the entrapped C: P and N: P ratios between consecutive trap samplings were negatively correlated with the time variations of the zooplankton C: P and N: P ratios. Zooplankton C: P and N: P ratios were positively correlated with the percentage of copepod biomass in total zooplankton biomass >250 µm and negatively correlated with the percentage of cladocerans. Zooplankton biomass >250 µm reduced the fraction of N and P primary production lost to sinking (export ratio). The residuals of the N export ratio versus zooplankton biomass relationship were negatively correlated with the zooplankton N: P ratio, whereas there was a positive relationship with the residuals of the P export ratio relationship. These observations support the hypothesis that the regulation of elemental homeostasis in the herbivorous zooplankton consumers occurs at least partly at the assimilation/egestion level. Elements ingested in excess—P for the herbivorous copepods and N for many cladocerans— are concentrated into sinking feces, whereas the deficient elements are captured into biomass.
Lead and copper speciation in remote mountain lakes
Authors: Plöger Anja , Fischer Eric , Nirmaier Hans-Peter , Laglera Luis M. , Monticelli Damiano , van den Berg Constant M.G.
Published: July 2010

We determined the chemical speciation of lead and copper in remote mountain and high‐latitude lakes at different times of the year, providing background data for regions in Europe least affected by anthropogenic effects. The lakes are characterized by low ionic strength, clear waters, and oligotrophic conditions; are fed predominantly by atmospheric precipitation; and are ice‐covered during a large part of the year. Lead—and copper‐complexing ligands were determined by cathodic stripping voltammetry with ligand competition using Calcein blue and salicylaldoxime, respectively. Water column averages of the dissolved lead concentrations ranged from 0.2 to 0.9 nmol L−1, with generally lowest levels during the prolonged winter ice cover, whereas dissolved copper concentrations, varying from 1 to 1.7 (±0.1) nmol L−1, showed no significant seasonal variations. The lead speciation was controlled by complexation with ligands at concentrations mostly below 2 nmol L−1, with values for the stability constant (log K’PbL) of 12.5‐13.7; calculated 9PbL ionic lead concentrations (‐log[Pb2+] values) were 11.9‐14.5. Copper complexation was controlled by ligands at concentrations of 12—21 nmol L−1, with values for log K’CuL of 13.5‐14.0 and—log[Cu2+] values of 14.4‐15.1. The concentration of the copper‐binding ligands, but not those for lead, varied seasonally, with about 50% higher concentrations during open water conditions compared to periods of ice cover. The data were consistent with the presence of only one class of ligands for copper and lead. The stability constant of the ligands is similar to that of fulvic acid; however, evidence regarding the actual nature of the ligands is still lacking. The lake data show that (1) competition between calcium and lead causes a reduction in the stability constant of approximately one log‐unit for each order of magnitude of Ca2+, and (2) lead scavenging in the lakes is moderated by complexation.