On the use of sediment fertilization for seagrass restoration: A mesocosm study on Zostera marina L.

The use of nitrogen and phosphorus sediment fertilization for seagrass restoration is explored. Special attention was given to the effects of nitrogen sediment fertilization. The sediment fertilization treatment combined different levels of nitrogen (0, 30, 500 mg N g DW^-1) with sufficient phosphorus to avoid P-limitation (fertilizer N:P ratio < 0.25). Using indoor mesocosms, we studied the effects of sediment fertilization, and its interactions with light availability (55, 200 μmol m^-2 s^-1) and sediment redox conditions (300, - 100mV), on Zostera marina L. We assessed (1) treatment effects on growth and plant biomass distribution, (2) the capacity of Z. marina roots to meet the plant nutrient demand, (3) plant tolerance to high nutrient porewater concentration, and (4) pro's and con's of use NH₄NO₃ as the N source in sediment fertilization for seagrass restoration. Plant biomass, growth and leaf turnover rate were stimulated by light and sediment fertilization. Biomass partitioning was not affected by light availability, whereas the relative root production was decreased in fertilized sediments. Root uptake following fertilization met nutrient plant demand. After high sediment fertilization, ammonium porewater concentration was high (30 mM) regardless of redox conditions. On the other hand, nitrate availability was also high, but 80% lower in reduced sediments (0.7-4 mM) compared to non-reduced ones (20 mM). Plants of Z. marina exhibited a remarkable tolerance to high N + P sediment fertilization. However, plant inhibition (reduction in plant weight, leaf growth and leaf turnover rate) was detected when porewater N concentrations exceeded 30 mM. The effects of phosphorus and ammonium toxicity were discarded because availability was similar for both inhibited and non-inhibited plants. We attributed the Z. marina inhibition to the extra porewater nitrogen available as nitrate (20 mM). Experimental treatments did not inhibit the photosynthetic apparatus of Z. marina. The mechanisms of inhibition might be related with deficiencies in energy or C-skeletons, since inhibitory effects were buffered when saturating irradiance and/or nitrate levels decreased in reduced sediments. In conclusion, we consider that the combined N + P sediment fertilization, with NH₄NO₃ as N source and high P supply, is highly beneficial for Z. marina restoration. This species has positive response to N + P sediment fertilization, high tolerance to the extensive porewater enrichment, and bacterial metabolism may reduce the porewater nitrate availability in anoxic seagrass sediments. However, for adequate sediment fertilization for restoration purposes, several precautions are suggested.