Changes in the pelagic microbial food web due to artificial eutrophication

Abstract

The effect of nutrient enrichment on the structure and carbon flow in the pelagic microbial food web was studied in mesocosm experiments using seawater from the northern Baltic Sea. The experiments included food webs of at least four trophic levels; (1) phytoplanktonbacteria, (2) flagellates, (3) ciliates and (4) mesozooplankton. In the enriched treatments high autotrophic growth rates were observed, followed by increased heterotrophic production. The largest growth increase was due to heterotrophic bacteria, indicating that the heterotrophic microbial food web was promoted. This was further supported by increased growth of heterotrophic flagellates and ciliates in the high nutrient treatments. The phytoplankton peak in the middle of the experiments was mainly due to an autotrophic nanoflagellate, Pyramimonas sp. At the end of the experiment, the proportion of heterotrophic organisms was higher in the nutrient enriched than in the nutrient-poor treatment, indicating increased predation control of primary producers. The proportion of potentially mixotrophic plankton, prymnesiophyceans, chrysophyceans and dinophyceans, were significantly higher in the nutrient-poor treatment. Furthermore, the results indicated that the food web efficiency, defined as mesozooplankton production per basal production (primary production + bacterial production -sedimentation), decreased with increasing nutrient status, possibly due to increasing loss processes in the food web. This could be explained by promotion of the heterotrophic microbial food web, causing more trophic levels and respiration steps in the food web.

Effects of catchment area and nutrient deposition regime on phytoplankton functionality in alpine lakes

“…Under higher P-deposition regime, it is likely that photoautotroph growth was less limited by inorganic P during the summer than in the northern lakes, leading to marked dominance by photoautotrophs in late summer whatever the catchment area. These findings are consistent with the widely observed trend for photoautotrophs to out-compete mixotroph algae after nutrient inputs (Andersson et al, 2006;Isaksson et al, 1999). The dominance of chlorophytes has already been reported under high trophic conditions in the Alps (Tolotti et al, 2006).…”

Influence of drought on algal biofilms and meiofaunal assemblages of temperate reservoirs and rivers

“…Chrysophyceae was a minor class in biofilms both in 2003 and 2005. The nutrient requirements of Chrysophyceae in biofilms have been scarcely studied but it was observed that the proportion of potentially mixotrophic planktonic Chrysophyceae was significantly higher in nutrientpoor treatments (Andersson et al, 2006).…”

Long-term Changes and Controlling Factors of Phytoplankton Community in the Gulf of Riga (Baltic Sea)

“…Nutrient enrichment increases the growth of most common phytoplankton taxonomic groups like diatoms, chlorophytes, and small autotrophic flagellates (Vuorio et al 2005;Andersson et al 2006), while N 2 -fixing cyanobacteria are favored by high or elevated P levels and low inorganic N/P ratios, usually below Redfield value (Kangro et al 2007;Vahtera et al 2007). The experimental studies have shown the importance of nutrient enrichment by proportional responses in primary production and by stimulating the dominance of specific phytoplankton species/group.…”