Dissolved organic matter (DOM) is the largest reservoir of reduced carbon in the oceans. The nature of DOM is poorly understood, in part, because it has been difficult to isolate sufficient amounts of representative material for analysis. Tangential-flow ultrafiltration was shown to recover milligram amounts of >1000 daltons of DOM from seawater collected at three depths in the North Pacific Ocean. These isolates represented 22 to 33 percent of the total DOM and included essentially all colloidal material. The elemental, carbohydrate, and carbon-type (by (13)C nuclear magnetic resonance) compositions of the isolates indicated that the relative abundance of polysaccharides was high ( approximately 50 percent) in surface water and decreased to approximately 25 percent in deeper samples. Polysaccharides thus appear to be more abundant and reactive components of seawater DOM than has been recognized.

Abstract. Compound-specific stable isotope analysis (CSIA) of individual amino acids (AAs) has become a powerful analytical tool in trophic ecology. Heavily fractionating "trophic" AAs (e.g., glutamic acid: Glu) provide a robust indicator of trophic transfer, while minimally fractionating "source" AAs (e.g., phenylalanine: Phe) closely reflect the δ 15 N value at the base of the food web (δ 15 N baseline ). Together, the CSIA-AA approach provides an unprecedented ability to disentangle the influences of δ 15 N baseline values and trophic fractionation on consumer nitrogen isotope values. Perhaps the most important assumption underlying CSIA-AA applications to trophic ecology is that trophic fractionation of Glu and Phe, and thus the trophic discrimination factor TDF Glu-Phe (Δ 15 N Glu − Δ 15 N Phe ), is effectively constant across diverse consumer-resource relationships. To test this assumption, we conducted a comprehensive meta-analysis of controlled feeding experiments that examined individual AA trophic fractionation (Δ 15 N C-D ) and resulting TDF Glu-Phe values. We found tremendous variability in TDF Glu-Phe values from 0‰ to >10‰ across 70 species (317 individuals) and 88 distinct consumer-diet combinations. However, this variability appears to follow predictable patterns driven by two dominant variables: diet quality and mode of nitrogen excretion. Consumers feeding on high-quality diets (small diet-consumer AA imbalances) tend to have significantly lower TDF Glu-Phe values than consumers feeding on low-quality diets. Similarly, urea/uric acid-producing consumers also exhibit significantly lower TDF Glu-Phe values than their ammonia-producing counterparts. While these patterns are certainly not universal, together these factors likely explain many of the observed patterns of TDF Glu-Phe variability. We provide an overview of the biochemical and physiological mechanisms underpinning AA Δ 15 N C-D to explain these patterns. There are several seemingly unique systems, including the remarkably consistent TDF Glu-Phe values across insect food webs and the isotopically "invisible" trophic transfers in microbial food webs, that may provide additional insight into the influence of diet quality and nitrogen cycling on AA fractionation. In this review, we argue that to realize the full potential of CSIA-AA approaches in trophic ecology, we must embrace the variability in TDF Glu-Phe values. This likely requires developing new models of trophic transfer dynamics for some applications, including multi-TDF Glu-Phe equations that directly incorporate variability in TDF Glu-Phe value.

Tracing the origin of nutrients is a fundamental goal of food web research but methodological issues associated with current research techniques such as using stable isotope ratios of bulk tissue can lead to confounding results. We investigated whether naturally occurring δ13C patterns among amino acids (δ13CAA) could distinguish between multiple aquatic and terrestrial primary production sources. We found that δ13CAA patterns in contrast to bulk δ13C values distinguished between carbon derived from algae, seagrass, terrestrial plants, bacteria and fungi. Furthermore, we showed for two aquatic producers that their δ13CAA patterns were largely unaffected by different environmental conditions despite substantial shifts in bulk δ13C values. The potential of assessing the major carbon sources at the base of the food web was demonstrated for freshwater, pelagic, and estuarine consumers; consumer δ13C patterns of essential amino acids largely matched those of the dominant primary producers in each system. Since amino acids make up about half of organismal carbon, source diagnostic isotope fingerprints can be used as a new complementary approach to overcome some of the limitations of variable source bulk isotope values commonly encountered in estuarine areas and other complex environments with mixed aquatic and terrestrial inputs.

During the early Pliocene, 5 to 3 million years ago, globally averaged temperatures were substantially higher than they are today, even though the external factors that determine climate were essentially the same. In the tropics, El Niño was continual (or "permanent") rather than intermittent. The appearance of northern continental glaciers, and of cold surface waters in oceanic upwelling zones in low latitudes (both coastal and equatorial), signaled the termination of those warm climate conditions and the end of permanent El Niño. This led to the amplification of obliquity (but not precession) cycles in equatorial sea surface temperatures and in global ice volume, with the former leading the latter by several thousand years. A possible explanation is that the gradual shoaling of the oceanic thermocline reached a threshold around 3 million years ago, when the winds started bringing cold waters to the surface in low latitudes. This introduced feedbacks involving ocean-atmosphere interactions that, along with ice-albedo feedbacks, amplified obliquity cycles. A future melting of glaciers, changes in the hydrological cycle, and a deepening of the thermocline could restore the warm conditions of the early Pliocene.

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