Composition and Transformation of Dissolved Organic Matter in the Baltic Sea

Abstract

The processing of terrestrial dissolved organic matter (DOM) in coastal shelf seas is an important part of the global carbon cycle, yet, it is still not well understood. One of the largest brackish shelf seas, the Baltic Sea in northern Europe, is characterized by high freshwater input from sub-arctic rivers and limited water exchange with the Atlantic Ocean via the North Sea. We studied the molecular and isotopic composition and turnover of solid-phase extractable (SPE) DOM and its transformation along the salinity and redox continuum of the Baltic Sea during spring and autumn. We applied ultrahigh-resolution mass spectrometry and other geochemical and biological approaches. Our data demonstrate a large influx of terrestrial riverine DOM, especially into the northern part of the Baltic Sea. The DOM composition in the central Baltic Sea changed seasonally and was mainly related to autochthonous production by phytoplankton in spring. Especially in the northern, river-dominated basins, a major fraction of riverine DOM was removed, likely by bio-and photo-degradation. We estimate that the removal rate of terrestrial DOM in the Baltic Sea (Bothnian Bay to the Danish Straits/Kattegat area) is 1.6-1.9 Tg C per year which is 43-51% of the total riverine input. The export of terrestrial DOM from the Danish Straits/Kattegat area toward the North Sea is 1.8-2.1 Tg C per year. Due to the long residence time of terrestrial DOM in the Baltic Sea (total of ca. 12 years), seasonal variations caused by bio-and photo-transformations and riverine discharge are dampened, resulting in a relatively invariant DOM molecular and isotopic signature exported to the North Sea. In the deep stagnant basins of the Baltic Sea, the DOM composition and dissolved organic nitrogen concentrations changed seasonally, likely because of vertical particle transport and subsequent degradation releasing DOM. DOM in the deep anoxic basins was also enriched in sulfur-containing organic molecules, pointing to abiotic sulfurization of DOM under sulfidic conditions.

Differential responses of marine, mesohaline and oligohaline bacterial communities to the addition of terrigenous carbon

“…A fundamental difference between the estuarine‐like Baltic Sea and river estuaries is the limited exchange of saltwater and freshwater in the Baltic Sea, resulting in long water residence times. Like most estuaries, the Baltic Sea, and especially its oligohaline northern part, contains a considerable amount of tDOM and nutrients (van Dongen et al ., ; Kuliński et al ., ; Deutsch et al ., ; Seidel et al ., ). For the period 1996–2000, the average DOC concentration for rivers draining into the Bothnian Bay and the Bothnian Sea was 498 and 623 µmol l −1 respectively (Deutsch et al ., ).…”

“…() and Seidel et al . (), who determined a high impact of tDOM on the food web, especially in the oligohaline Bothnian Sea. However, the importance of tDOM biodegradation is still debated because in addition to the catchment areas other environmental factors, such as turbidity (Obernosterer and Benner, ) and salinity (Asmala et al ., ), seem to play a role.…”

“…Because the northern Baltic Sea is dominated by sub‐arctic wetlands and forests, its water chemistry is comparable to that of larger western Russian Arctic rivers (Humborg et al ., ; Alling et al ., ). The high river‐water discharge in the northern Baltic Sea results in a tDOM‐dominated (up to 90% of terrestrial origin) DOC pool in the northern Baltic Sea, unlike in the western Baltic Sea (Alling et al ., ; Deutsch et al ., ; Fleming‐Lehtinen et al ., , Seidel et al ., ). However, the DOC concentration in rivers is often still much higher than in coastal waters (Hoikkala et al ., ), due to removal processes such as biodegradation (Wikner et al ., ; Sandberg et al ., ; Logue et al ., ).…”

Large-Scale Nutrient Dynamics in the Baltic Sea, 1970–2016

“…Humic substances have a higher proportion of nitrogen; easily degradable "organic" fractions have a higher proportion of phosphorus (Nausch and Nausch, 2011), part of which, in oxic conditions, can be associated with iron-humic complexes as well as phosphates adsorbed on particles (e.g., Ylöstalo et al, 2016;Li et al, 2017). Unfortunately, the existing information on organic nutrients, especially on their degradability is rather fragmentary (e.g., Hoikkala et al, 2015;KnudsenLeerbeck et al, 2017;Seidel et al, 2017). Therefore, first-order estimates of the degradable fraction of organic nutrients were made as follows.…”

“…Although such estimates should be considered as rather simplified, semi-qualitative approximations, the time depth variations obtained look rather plausible (see examples in Supplementary Material): the lower values are found in the deeper parts of upper layers, while in the surface layers the effects of extracellular excretion from spring and summer phytoplankton blooms, followed by sedimentation and lysis of degrading seston, could be recognized with a little imagination. The main result of this large-scale exercise, performed in every basin on thousands of measurements, is similar to deductions TP pool, 10 3 t P 9.5 ± 1.8 64.9 ± 14.7 462 ± 70 25.4 ± 6.1 10.4 ± 1.7 11.5 ± 2.2 10.8 ± 1.5 595 ± 88 TP input, 10 3 t P yr −1 3.4 ± 0.7 4.0 ± 0.7 24.2 ± 5.6 9.0 ± 2.2 3.9 ± 1.4 4.8 ± 3.0 2.5 ± 0.7 51.8 ± 12.5 TN:TP, mol:mol 94 ± 20 40 ± 7 17 ± 2 30 ± 7 33 ± 6 23 ± 4 23 ± 4 21 ± 3 TN in :TP in , mol:mol 41 ± 6 51 ± 6 52 ± 5 34 ± 10 64 ± 20 57 ± 28 83 ± 18 50 ± 6 DIN, 10 3 t N 124 ± 23 196 ± 58 620 ± 131 60 ± 24 30 ± 11 21 ± 6 27 ± 8 1,077 ± 210 DIP, 10 3 t P 2.6 ± 0.8 35.7 ± 13.6 348 ± 48 15.2 ± 3.9 3.9 ± 1.2 5.3 ± 1.5 5.4 ± 1.0 416 ± 62 DIN:DIP, mol:mol 119 ± 46 13 ± 3 4 ± 1 9 ± 3 19 ± 10 9 ± 3 11 ± 4 6 ± 1 Mean ± standard deviations from pools (1970-2016) and inputs (1970-2014) made from scarcer data (Hoikkala et al, 2015;Seidel et al, 2017): the share of the labile fraction of organic nitrogen (21-37%) increases toward the Baltic Entrance area and is significantly lower than that of phosphorus (35-67%).…”