Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L⁻¹; median: 103 µmol C L⁻¹) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmol N L⁻¹; median: 8.0 µmol N L⁻¹), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmol P L⁻¹; median: 0.18 µmol P L⁻¹). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

To obtain qualitative information on the Black Sea's dissolved organic matter (DOM) pool, the optical properties (absorption and fluorescence) were measured in 111 samples collected across the basin. Chromophoric dissolved organic matter (CDOM) was considered at three wavelengths (254 nm, 280 nm, and 325 nm), along with the spectral slope between 275 nm and 295 nm (S_275–295) and the specific ultraviolet absorbance at 254 nm (SUVA₂₅₄). Using parallel factor analysis, a five‐component model identified three humic‐like components, a protein‐like component, and a polycyclic aromatic hydrocarbon‐like component. In the basin's oxic layer (upper ∼ 100 m), protein‐like CDOM was elevated, likely due to the production of this labile material, while humic‐like material was low, suggesting its removal by photo‐oxidation. In the underlying waters, the protein‐like material decreased, perhaps due to the utilization of this nitrogen‐containing DOM, while humic‐like material increased, suggesting its production at depth. In the anoxic layer (lower ∼ 2000 m), dissolved organic carbon (DOC) varied by only ∼ 10% while CDOM increased with depth by a factor of approximately two; the optical properties correlated well with the H₂S‐equivalence of mineralization, referred to here as apparent carbon mineralization (ACM), while DOC did not. The strong correlation between CDOM and ACM is similar to correlations previously identified in the open ocean that compared CDOM with apparent oxygen utilization, suggesting that CDOM accumulates as a function of mineralization, independently of the oxidizing agent (i.e., oxygen or sulfate).

Exudates by the diatom Phaeodactylum tricornutum were incubated with a natural community of marine heterotrophic prokaryotes for 24 d in order to investigate the link between the biological lability and the molecular weight, fluorescence, and polarity of phytoplankton dissolved organic matter (DOM). Dissolved organic carbon (DOC) removal, changes in fluorescence and in the heterotrophic prokaryote abundance were followed over time both in the total exudates and in the low‐ and high‐molecular‐weight fractions. To detect changes in the polarity of proteins, reverse‐phase high‐performance liquid chromatography (HPLC) was applied to the high‐molecular‐weight fraction. Our results indicate that freshly produced phytoplankton DOM exhibits a dynamic pattern of degradation that is accompanied by large changes in the growth efficiency of the bacterial community that are likely related to changes in DOM quality. Approximately 20% of high‐molecular‐weight DOM and 40% of fluorescence attributed to protein‐like DOM were degraded over the first days of the incubation indicating that protein‐like DOM is likely a labile component of phytoplankton exudates. In contrast, fluorescence measurements suggest that humic‐like substances are resistant to bacterial degradation over the 24 d of the experiment. Despite fluctuations in the short‐term rates of high‐molecular‐weight and low‐molecular‐weight DOM removal, the relative contributions of these fractions to DOM pool were similar in the fresh exudates and at the end of our incubation experiments.