DOC removal paradigms in highly humic aquatic ecosystems
- 570 Downloads
Background, aim, and scope
Dissolved humic substances (HS) usually comprise 50–80% of the dissolved organic carbon (DOC) in aquatic ecosystems. From a trophic and biogeochemical perspective, HS has been considered to be highly refractory and is supposed to accumulate in the water. The upsurge of the microbial loop paradigm and the studies on HS photo-degradation into labile DOC gave rise to the belief that microbial processing of DOC should sustain aquatic food webs in humic waters. However, this has not been extensively supported by the literature, since most HS and their photo-products are often oxidized by microbes through respiration in most nutrient-poor humic waters. Here, we review basic concepts, classical studies, and recent data on bacterial and photo-degradation of DOC, comparing the rates of these processes in highly humic ecosystems and other aquatic ecosystems.
Materials and methods
We based our review on classical and recent findings from the fields of biogeochemistry and microbial ecology, highlighting some odd results from highly humic Brazilian tropical lagoons, which can reach up to 160 mg C L−1.
Results and discussion
Highly humic tropical lagoons showed proportionally lower bacterial production rates and higher bacterial respiration rates (i.e., lower bacterial growth efficiency) than other lakes. Zooplankton showed similar δ13C to microalgae but not to humic DOC in these highly humic lagoons. Thus, the data reviewed here do not support the microbial loop as an efficient matter transfer pathway in highly humic ecosystems, where it is supposed to play its major role.
In addition, we found that some tropical humic ecosystems presented the highest potential DOC photo-chemical mineralization (PM) rates reported in the literature, exceeding up to threefold the rates reported for temperate humic ecosystems. We propose that these atypically high PM rates are the result of a joint effect of the seasonal dynamics of allochthonous humic DOC input to these ecosystems and the high sunlight incidence throughout the year. The sunlight action on DOC is positive to microbial consumption in these highly humic lagoons, but little support is given to the enhancement of bacterial growth efficiency, since the labile photo-chemical products are mostly respired by microbes in the nutrient-poor humic waters.
HS may be an important source of energy for aquatic bacteria in humic waters, but it is probably not as important as a substrate to bacterial growth and to aquatic food webs, since HS consumption is mostly channeled through microbial respiration. This especially seems to be the case of humic-rich, nutrient-poor ecosystems, where the microbial loop was supposed to play its major role. Highly humic ecosystems also present the highest PM rates reported in the literature. Finally, light and bacteria can cooperate in order to enhance total carbon degradation in highly humic aquatic ecosystems but with limited effects on aquatic food webs.
Recommendations and perspectives
More detailed studies using C- and N-stable isotope techniques and modeling approaches are needed to better understand the actual importance of HS to carbon cycling in highly humic waters.
KeywordsBacterioplankton Coastal lagoons Dissolved humic substances Dissolved organic carbon Humic ecosystems Microbial loop Photochemical mineralization Photo-degradation Photo-oxidation
- Amado AM, Farjalla VF, Esteves FA, Bozelli RL (2003) DOC photo-oxidation in clear water Amazonian aquatic ecosystems. Amazoniana 17:513–523Google Scholar
- Bertilsson S, Tranvik LJ (2000) Photochemical transformation of dissolved organic matter in lakes. Limnol Oceanogr 45:753–762Google Scholar
- Daniel C, Granéli W, Kritzberg ES, Anesio AM (2006) Stimulation of metazooplankton by photochemically modified dissolved organic matter. Limnol Oceanogr 51:101–108Google Scholar
- del Giorgio PA, Davis J (2003) Patterns in dissolved organic matter lability and consumption across aquatic ecosystems. In: Findlay SEG, Sinsabauhg RL (eds) Aquatic Ecosystems: Interactivity of dissolved organic matter. Academic Press, San Diego, CA, pp 399–424Google Scholar
- Fry B (2006) Stable Isotope Ecology. Springer, New York, NY, USAGoogle Scholar
- Granéli W, Lindell M, Tranvik L (1996) Photo-oxidative production of dissolved inorganic carbon in lakes of different humic content. Limnol Oceanogr 41:698–706Google Scholar
- Hollanda-Carvalho P, Sánchez-Botero JI, Pellegrini-Caramaschi E, Bozelli RL (2003) Temporal variation of fish community richness in coastal laggons of Restinga de Jurubatiba National Park, Rio de Janeiro, Brazil. Acta Limnol Bras 15:31–40Google Scholar
- Jansson M (1998) Nutrient limitation and bacteria-phytoplankton interactions in humic lakes. In: Hessen DO, Tranvik L (eds) Aquatic Humic Substances–Ecology and Biogeochemistry. Springer-Verlag, Berlin, pp 177–195Google Scholar
- Jansson M, Blomqvist P, Jonsson A, Bergström A-K (1996) Nutrient limitation of bacterioplankton, autotrophic and mixotrophic phytoplankton and heterotrophic nanoflagellates in Lake Örträsket, a large humic lake in northern Sweden. Limnol Oceanogr 41:1552–1559Google Scholar
- Jansson M, Bergström A-K, Blomqvist P, Drakare S (2000) Allochthonous organic carbon and phytoplankton/bacterioplankton production relationships in lakes. Ecology 81:3250–3255Google Scholar
- Jonsson A, Meili M, Bergstrom A-K, Jansson M (2001) Whole-lake mineralization of allochthonous and autochthonous organic carbon in a large humic lake (Ortrasket, N. Sweden). Limnol Oceanogr 46:1691–1700Google Scholar
- Karlsson J, Jonsson A, Meili M, Jansson M (2003) Control of zooplankton dependence on allochthonous organic carbon in humic and clearwater lakes in northern Sweden. Limnol Oceanogr 48:269–276Google Scholar
- Lindell MJ, Granéli W, Tranvik LJ (1995) Enhanced bacterial growth in response to photochemical transformation of dissolved organic matter. Limnol Oceanogr 40:195–199Google Scholar
- McKnight DM, Aiken GR (1998) Sources and age of aquatic humus. In: Hessen DO, Tranvik L (eds) Aquatic Humic Substances–Ecology and Biogeochemistry. Springer-Verlag, Berlin, pp 9–39Google Scholar
- Moran MA, Covert JS (2003) Photochemical mediated linkages between dissolved organic matter and bacterioplankton. In: Findlay SEG, Sinsabauhg RL (eds) Aquatic Ecosystems: Interactivity of dissolved organic matter. Academic Press, San Diego, CA, pp 243–262Google Scholar
- Münster U, HaanH De (1998) The role of microbial extracellular enzymes in the transformation of dissolved organic matter in humic waters. In: Hessen DO, Tranvik L (eds) Aquatic Humic Substances–Ecology and Biogeochemistry. Springer-Verlag, Berlin, pp 199–257Google Scholar
- Steinberg CEW (2003) Ecology of humic substances in freshwaters. Springer, BerlinGoogle Scholar
- Suhett AL, MacCord F, Amado AM, Farjalla VF, Esteves FA (2004) Photodegradation of dissolved organic carbon in humic coastal lagoons (Rio de Janeiro, Brazil). In: Proceedings of the XII Meeting of the International Humic Substances Society, São Pedro, SP, Brasil. pp 61–63Google Scholar
- Thurman EM (1985) Organic geochemistry of natural waters. Junk Publishers, Dordrecht, Dr WGoogle Scholar
- Tranvik L (1998) Degradation of dissolved organic matter in humic waters by bacteria. In: Hessen DO, Tranvik L (eds) Aquatic Humic Substances–Ecology and Biogeochemistry. Springer-Verlag, Berlin, pp 259–283Google Scholar
- Vadstein O (2000) Heterotrophic, planktonic bacteria and cycling of phosphorus-phosphorus requirements, competitive ability, and food web interaction. In: Schink B (ed) Advances in Microbial Ecology. Kluwer Academic Publishers, New York, pp 115–167Google Scholar