Efficiency of the coastal filter: Nitrogen and phosphorus removal in the Baltic Sea

Abstract

An important function of coastal ecosystems is the reduction of the nutrient flux from land to the open sea, the coastal filter. In this study, we focused on the two most important coastal biogeochemical processes that remove nitrogen and phosphorus permanently: denitrification and phosphorus burial. We compiled removal rates from coastal systems around the Baltic Sea and analyzed their spatial variation and regulating environmental factors. These analyses were used to scale up denitrification and phosphorus burial rates for the entire Baltic Sea coastal zone. Denitrification rates ranged from non‐detectable to 12 mmol N m−2 d−1, and correlated positively with both bottom water nitrate concentration and sediment organic carbon content. The rates exhibited a strong decreasing gradient from land to the open coast, which was likely driven by the availability of nitrate and labile organic carbon, but a high proportion of non‐cohesive sediments in the coastal zone decreased the denitrification efficiency relative to the open sea. Phosphorus burial rates varied from 0.21 g P m−2 yr−1 in open coastal systems to 2.28 g P m−2 yr−1 in estuaries. Our analysis suggests that archipelagos are important phosphorus traps and account for 45% of the coastal P removal, while covering only 17% of the coastal areas. High burial rates could partly be sustained by phosphorus import from the open Baltic Sea. We estimate that the coastal filter in the Baltic Sea removes 16% of nitrogen and 53% of phosphorus inputs from land.

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

“…There, basin-wise nitrogen and phosphorus coastal retention were provided from the compilation by Sjoberg et al (2002). Fifteen years later, a new compilation was made by Asmala et al (2017). These estimates of nutrient removal in the coastal zone by denitrification and phosphorus burial could be subtracted directly from the total basin-wise sinks calculated here (cf.…”

“…Such transport is even projected to increase in the future due to reduced terrestrial loading and enhanced phosphorus bottom release in warmer environments Almroth-Rosell et al, 2016). Still, the simulated annual phosphate imports into the coastal areas are on the order of hundreds, rather than thousands, of tons, as suggested by Asmala et al (2017).…”

A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century

“…Yet, although long-term trends in the expansion of hypoxia in offshore areas of the Baltic Sea have been widely studied, little is known about the past evolution of hypoxia in the shallow coastal areas, where episodic or seasonal oxygen deficiency is forced by thermal rather than salinity stratification (Virtasalo et al, 2005;Conley et al, 2011). Importantly, these coastal areas act as a filter for nutrients received from the catchment (Asmala et al, 2017). Thus, changes in biogeochemical cycles in coastal sediments may have an impact on nutrient transport to offshore areas of the Baltic Sea (Almroth-Rosell et al, 2016).…”

Factors Controlling Storage, Sources, and Diagenetic State of Organic Carbon in a Prograding Subaerial Delta: Wax Lake Delta, Louisiana

“…Coastal wetlands reduce nutrient loads to the ocean (Asmala et al, 2017;Breaux et al,1995;Mitsch et al, 2001), provide storm surge protection (Day et al, 2007;Laska et al, 2005), and sequester CO 2 from the atmosphere (Chmura et al, 2003). On a per unit area basis, these systems sequester carbon at rates that are an order of magnitude greater than boreal, tropical, and temperate forest ecosystems, (McLeod et al, 2011;Shields et al, 2017).…”