Nutrient and chlorophyll a temporal patterns in eutrophic mountain ponds with contrasting macrophyte coverage.

In shallow lakes, macrophytes have important effects on food webs, community structure and nutrient dynamics. For this reason they play a significant role in the restoration of degraded systems. Particularly when present with large coverage, they establish mechanisms favoring low phytoplankton biomass and a decrease of nutrient concentration in the water column. Even if these effects are documented from a number of systems, little evidence comes from shallow mountain lakes. In this study, we report temporal patterns of phytoplanktonic chlorophyll a and nutrient concentration in six eutrophic mountain ponds showing contrasting macrophyte coverage. Six ponds, situated in the Italian Apennines at altitudes ranging from 1164 to 1604 m, were monitored from March to November 1997. Four subsurface water samples were collected at each pond on every visit and analysed for chlorophyll a and nutrients following standard methods. Ponds were divided into two groups: vegetated (VP) and unvegetated (UP) according to their summer macrophyte coverage (respectively > 30% and < 30% of pond area). A nested ANOVA was used to detect macrophyte coverage, season and date (nested within season) effect on phytoplanktonic chlorophyll a concentration, dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP). Chlorophyll a was affected by coverage (P < 0.01) and by season (P < 0.05). In both groups chlorophyll a concentration increased from spring to summer, but differences were higher in UP than in VP. DIN was affected by coverage and season but temporal pattern was not consistent among UP and VP (interaction among coverage and season was significant, P < 0.01). SRP was not affected by coverage (P = 0.158) and season (P = 0.169). Our results on mountain ponds are consistent with those of similar lowland systems. We conclude that managing the presence of macrophytes can be an effective tool for the restoration of eutrophic shallow lakes also in mountain areas.