Effects of water chestnut (Trapa natans) beds on water chemistry in the tidal freshwater hudson river

Vegetated areas of rivers and estuaries are capable of affecting the concentration of dissolved and particulate matter in water masses traversing those plant beds. We examined whether different sizes of water chestnut (Trapa natans) beds in the Hudson River, USA, alter dissolved oxygen, nutrients and turbidity of water masses. Ebb-tide water was sampled from four water chestnut beds in the tidal freshwater portion of the Hudson River estuary and each site was sampled multiple times during the growing season and once following plant senescence. Water quality variables included dissolved oxygen, turbidity, dissolved organic carbon, and inorganic nutrients. Samples from the small beds (575 m(2) and 624 m(2)) were compared with large beds (16 600 m(2) and 24 820 m(2)). Dissolved oxygen of water flooding vegetated beds in the hour before high tide was 7.18 +/- 1.03 mg/l (mean +/- standard deviation) with a range of 5.5-9.8 mg/l throughout the growing season. Water samples collected as water ebbed from the plant beds showed that only the large beds had an effect on dissolved oxygen with the largest declines in oxygen exhibited by the largest bed. Decline of dissolved oxygen in the water ebbing from the largest bed averaged 1.5 +/- 0.4 mg/l/h with a minimum of 4.5 mg/l, equivalent to 54% of saturation, a level at which sensitive fauna are negatively affected. There were no significant relationships between bed size or plant presence and inorganic nutrients, turbidity or DOC. Ebb-tide nitrate was never lower than 87% of flood tide means. Effective management of invasive plants must consider both the variability in effects among plant beds and the areal coverage of plant bed sizes.