MEPS 152:89-102 (1997)  -  doi:10.3354/meps152089

Experiments were designed to test the hypothesis that the response of a submersed plant community will differ with varying form, mode and timing of nutrient delivery. Two separate experiments were conducted in summer-fall 1993 and spring-summer 1994; each ran for a period of 12 wk. Dissolved and particulate forms of nutrient additions were made to 10l microcosms containing a submersed vascular plant species (Potamogeton perfoliatus L.) formally abundant in brackish waters of the Chesapeake Bay, USA. Nutrients in the pulse treatments were added to each of 4 replicate chambers weekly in fall and bi-weekly in spring, while continuous inputs were administered via a peristaltic pump. Additions were made at a moderate loading rate of 38 µmol N l ^-1d^-1 in both spring-summer and summer-fall experiments, and at a high loading rate of 5 times higher in spring-summer. Phosphorus was added to result in a 10:1 atomic N:P ratio. Results from the experiments involving moderate enrichment with dissolved nutrients indicate that water column concentrations remained low (1 to 2 µmol), while enrichment at the high levels exhibited an increase in concentrations over controls. Plant growth and biomass decreased with both moderate and high dissolved nutrient treatments, while morphological parameters also indicated plant stress. Algal concentrations, particularly epiphytes, increased with experimental treatment. Both plant and microalgal C:N ratios decreased with nutrient enrichment. In summer-fall, particulate organic nutrient (PON) enrichment at moderate levels resulted in a stimulation of plant growth in comparison to control. In spring-summer, PON enrichment resulted in initial stimulation of growth, followed by a decline. Total ecosystem production (P_a) and respiration (R_n) increased in experimental chambers. A nitrogen budget indicates a redirection of nutrients from plant biomass to algal biomass in the nutrient enriched systems. A seasonal comparison is made between the results from the summer-fall and spring-summer experiments. The relationship between water quality parameters in experimental chambers and established submersed plant habitat criteria is discussed.

Submersed aquatic vegetation · Mesocosms · Nutrient enrichment