Seasonal changes are also clearly evident in the dependence of DOC concentration on time in the course of a year (Figure 5). In the non-growing
season, DOC concentrations do not exceed 3.5 mg dm− 3 while in the growing season they reach as much as 8.2 mg dm− 3. This supports the conclusion that here are two pools of dissolved organic substances, labile and resistant to biochemical oxidation. The labile fraction of DOC is supplied to seawater in the period of intensive primary production, whereas the stable form persists in seawater throughout the year. Fluctuations of DOC and POC in Baltic seawater were reported by Jurkovskis et al. (1976), Pempkowiak et al. (1984), Grzybowski & Pempkowiak (2003), Burska (2005) and Woźniak (2014), while Kuliński & Pempkowiak (2008) suggested the existence of two DOC fractions of varying biochemical stability. It has been speculated throughout this text that both DOC UMI-77 chemical structure and POC concentrations
are influenced by the activity of plankton. The idea is firmly established in the literature (Thomas and Schneider, 1999, Hagström Dabrafenib molecular weight et al., 2001, Stoń et al., 2002, Doney et al., 2003, Thomas et al., 2005, Sarmiento and Gruber, 2006 and Segar, 2012). Also zooplankton can influence organic carbon concentrations in seawater (Dzierzbicka-Głowacka et al. 2011). The abundance of plankton can be approximated by proxies: chlorophyll a, phaeopigment a ( Bianchi et al., 1996, Meyer-Harms and von Bodungen, 1997, Wasmund and Uhlig, 2003 and Collos et al., 2005), while the phytoplankton activity influences the pH of seawater ( Edman & Omstedt 2013). To find answers to questions regarding the factors influencing POC and DOC concentrations, chlorophyll a (Chl a) and phaeopigment
a (Feo) concentrations, pH and temperature of seawater were measured simultaneously with DOC and POC. The measured water properties were used as proxies of phytoplankton abundance (Chl a), photosynthetic see more activity of phytoplankton (pH), activity of zooplankton (Feo), and season (Temp) ( Voipio, 1981, Omstedt and Axell, 2003, Schneider et al., 2003 and Kuliński and Pempkowiak, 2008) The relationships between the concentrations of DOC and POC are presented in Figure 6. They are characterised by a coefficient of determination R2 = 0.61, which gives a coefficient of correlation R = 0.78. This strong correlation can be attributed to the composition of POM, comprising both phyto- and zooplankton – direct sources of DOC, and to the bacterial disintegration of detritus ( Hoikkala et al. 2012), also a component of POM ( Dzierzbicka-Głowacka et al. 2011). The relationships between DOC and POC and the other individual factors – chlorophyll a, phaeopigment a, pH and temperature (combined results for the Gdańsk, Gotland and Bornholm Deeps) – are presented in Figures 7 and 8 for DOC and POC respectively. The dependences were approximated by linear equations. The slope coefficients and coefficients of determination (R2) are listed in Table 5.