Charles Y. Robertson

Controls on carbon isotope fractionation by diatoms in the Peru upwelling region

The fractionation of carbon isotopes during photosynthesis by phytoplankton is quantified for samples of suspended material collected along two transects across the Peru continental margin in 1992. The magnitude of fractionation is estimated using the δ13C of 24-methylcholesta-5,24(28)-dien-3β-ol (diatoms) and compared to that of C37.2 alkenone (haptophytes). Isotopic fractionation by diatoms exhibits a wide range and large scatter when plotted against the reciprocal of the concentration Of CO2(aq), while a strong correlation is observed for fractionation by alkenone-bearing haptophytes. Diatom growth rates, calculated from silicate concentrations and assuming Monod growth kinetics, normalized to [CO2(aq)] are well correlated to diatom fractionation factors. These results support the concept that growth rates, in addition to CO2 concentrations, impose a control on the fractionation of carbon isotopes by both taxonomic groups of algae. In addition, the very small fractionation factors for diatoms indicate that species in the Peru upwelling region employed mechanisms which actively transport inorganic carbon into cells. A size dependence is observed for the δ13C of the diatom sterol: 24-methylcholesta-5,24(28)-dien-3β-ol is enriched in 13C in samples of suspended material > 20 μm relative to the <20-μm fraction. This suggests that surface-area-to-volume ratios also impose a control on the fractionation of carbon isotopes by diatoms, a proposition that is supported by detailed cell geometry and isotopic data for two larger size fractions from one sample.

Composition, productivity and nutrient chemistry of a coastal ocean planktonic food web

A 3 year field study was conducted to investigate patterns, magnitude and variability of primary production; the abundance, biomass and composition of producers and consumers; and the relative importance of physical and chemical variables associated with these parameters, in inner shelf waters of the South Atlantic Bight. Discrete interval, time series and continuous measurements were made along a transect and at two process-oriented stations during summer and winter 1985–1988. A quasi-permanent density front constrains low salinity (<34‰) waters to within ca 10 km of the coast. These waters contain abundant autotrophic and heterotrophic communities. Primary production is high, 6–7 × 102gC m−2 year−1, and is apparently subsidized by rapid nutrient recycling in the water column, sediments and adjacent salt marshes. Silicate is notable for its excess concentrations year-round and supports substantial diatom productivity. Correlation analyses suggest that Si strongly influences phytoplankton biomass, whereas growth rates are coupled to availability of light and NH4. Despite evidence of considerable variability in primary production over daily to interannual scales, plankton biomass is relatively constant. Experimental studies suggest a tight coupling between primary producers and microconsumers, and support the hypothesis that substantial fractions of primary and secondary production are recycled within the water column.

Spring recycling rates of ammonium in turbid continental shelf waters off the southeastern United States

Abstract Nitrogen ( NH 4 + ) recycling rates were measured over 3 weeks and 1000 km 2 during April 1985 as part of a multidisciplinary study to determine the fate of low-salinity water off the southeastern coast of the United States. During the study period, low-salinity water was not advected offshore as expected but held against the coast by a weak onshore-southward wind stress. Several analytical nitrogen ( 15 N) models were examined to determine the appropriateness of each model to describe the experimental field data. Two frequently applied mathematical models were compared, and it was determined that Laws (1985, Limnology and Oceanography , 30 , 1340–1350) nitrogen cycling model best described our experimental data set. Mean values for NH 4 + regeneration at each station during the study did not vary by more than a factor of 3 from the overall mean value (0.224 ± 0.041μM h −1 , ±1 S.E.). NH 4 + utilization (0.111 ± 0.030 μM h −1 ) showed the same overall variance, i.e. less than a factor of 3. NH 4 + regeneration exceeded utilization by a factor of 1.7:1 ( U : R ratio was 0.58:1). NH 4 + assimilation, on the other hand, showed a significant decrease across the area (0.090-0.021 μM h −1 ) and correlated with particulate N and chlorophyll a . Regeneration rates were unaffected and assimilation rates were significantly affected (mean 75%) when coastal plankton were deprived of full sunlight. Nanoplankton and microplankton (1 μm) were responsible for most of the regeneration and assimilation of NH 4 + . In addition, NH 4 + disappearance from the NH 4 − phase was not balanced by an increase in particulate N, and NH 4 + assimilation contributed only 10–20% of the phytoplankton nitrogen requirement. These results suggest that phytoplankton are not the sole consumer of regenerated NH 4 + , the fate(s) of regenerated NH 4 + remains unknown and the spring phytoplankton off the southeastern United States may have been nitrogen limited.

Performance of cages as large animal-exclusion devices in the deep sea

Sedimentary, deep-sea communities include megafaunal animals (e.g., sea cucumbers, brittle stars, crabs) and demersal fishes, collectively termed the large, motile epifauna (LME). Individuals of the LME are common, and their biomass approximates that of the macrofauna. Based on analogies with shallow-water animals, they are likely to be sources of mortality for the infauna and to create spatial and temporal heterogeneity in the community. Given present theories of deep-sea community organization, such effects could be important. Unfortunately, this hypothesis has not been tested because of the difficulty of conducting experiments in the deep sea and because tools for manipulating the LME have not been developed. We studied the suitability of exclusion cages for this purpose at 780 m depth in San Diego Trough. We placed 16 cages of two mesh sizes for 4.5 months over regions of the seafloor that appeared free of LME. Time-lapse photographs of a cage and a control plot coupled with observations of all cages at the end of the experiment indicated that small (1.27-cm X 1.27-cm square)-mesh cages were effective at excluding LME. Further, the cages were essentially free of cage artifacts that have been reported in shallow-water studies. Large, mobile and disruptive animals (e.g., fishes, crabs) did not establish long-term residence adjacent to or on the cages. Bio-fouling slightly reduced the open surface area of the cage mesh, potentially reducing flow through the cage, but the composition of surface sediments in terms of organic C and N, phytoplankton-derived pigments, and grain size was indistinguishable between cages and control areas. Activities of excess 234 Th were significantly higher (average = 37%) inside of small-mesh cages, which might suggest enhanced particulate deposition inside cages. However, this measurement was an artifact of experimental manipulation. Particles that accumulated on the cage during the experiment were dislodged and settled to the seafloor when the cage was opened just prior to sampling. These particles would have been highly enriched in 234 Th, and their inclusion in core samples artificially inflated the calculated sediment accumulation rates inside cages. Therefore, the cages performed well; they excluded the targeted LME without causing artifacts and thus should be useful for experimental study of a group of animals that may have substantial impact on the structure and organization of deep-sea communities.

A Non-Native Prey Mediates the Effects of a Shared Predator on an Ecosystem Service

Non-native species can alter ecosystem functions performed by native species often by displacing influential native species. However, little is known about how ecosystem functions may be modified by trait-mediated indirect effects of non-native species. Oysters and other reef-associated filter feeders enhance water quality by controlling nutrients and contaminants in many estuarine environments. However, this ecosystem service may be mitigated by predation, competition, or other species interactions, especially when such interactions involve non-native species that share little evolutionary history. We assessed trophic and other interference effects on the critical ecosystem service of water filtration in mesocosm experiments. In single-species trials, typical field densities of oysters (Crassostrea virginica) reduced water-column chlorophyll a more strongly than clams (Mercenaria mercenaria). The non-native filter-feeding reef crab Petrolisthes armatus did not draw down chlorophyll a. In multi-species treatments, oysters and clams combined additively to influence chlorophyll a drawdown. Petrolisthes did not affect net filtration when added to the bivalve-only treatments. Addition of the predatory mud crab Panopeus herbstii did not influence oyster feeding rates, but it did stop chlorophyll a drawdown by clams. However, when Petrolisthes was also added in with the clams, the clams filtered at their previously unadulterated rates, possibly because Petrolisthes drew the focus of predators or habituated the clams to crab stimuli. In sum, oysters were the most influential filter feeder, and neither predators nor competitors interfered with their net effect on water-column chlorophyll. In contrast, clams filtered less, but were more sensitive to predators as well as a facilitative buffering effect of Petrolisthes, illustrating that non-native species can indirectly affect an ecosystem service by aiding the performance of a native species.