Effects of CO 2 enrichment on mineral accumulation and nitrogen relations in a submersed macrophyte

1. Six‐ to eight‐week greenhouse experiments with independent control of pH and dissolved CO 2 evaluated the potential for CO 2 enrichment to stimulate the accumulation of Al, Fe, P and N in shoots of Vallisneria americana , particularly at pH 5. These minerals were provided only as they occurred in natural lake sediments. 2. The effect of CO 2 enrichment at pH 5 v pH 7.3 on growth and tissue N concentration was also determined. 3. CO 2 enrichment at pH 5 effected 5.5‐ and 7‐fold increases in total shoot accumulation of Al and Fe, respectively. In a two‐way factorial experiment, CO 2 enrichment yielded 6‐ to 11‐fold greater total shoot P accumulation in plants grown on less and more fertile sediments, respectively. 4. In a three‐way factorial experiment, CO 2 enrichment stimulated Vallisneria growth, especially at pH 5, and resulted in a 31–58% reduction in tissue [N] for different pH × sediment combinations. These are greater reductions than previously reported. It also increased total shoot N accumulation up to 6‐fold, and there were significant interactions with pH and sediment source: the CO 2 enrichment effect on shoot N accumulation was greater at pH 5 than at pH 7.3, and it was greater with the more fertile sediment at pH 5. 5. Water chemistry (pH and/or [CO 2 ]) and sediment fertility thus both indirectly influenced the accumulation of sediment‐derived minerals in macrophyte shoots within the water column.