Assessment of the effectiveness of rearing marshes by planting bioengineering grasses of the genus Spartina in the Lagoa dos Patos estuary, RS, Brazil

Author: Adriana Martins Guedes de Azevedo (Currículo Lattes)
Supervisor: Dr César Serra Bonifácio Costa


Among the vascular aquatic plants of salinized environments, the marshes plants (coastal saline baths in the intertidal zone) are the most cultivated and planted by man, particularly for the purpose of establishing natural barriers against coastal erosion and the recovery of degraded areas. to variations in salinity, periodic submersion and hypoxic soil conditions, they are able to build their own environment by forming biogenic structures. The efficiency of the everglades created to reproduce the structural and functional attributes of natural everglades is still questioned, particularly if autogenic processes are capable of maintaining the development of everglades under high intensity of allogeneic forcing.This thesis aimed to evaluate the temporal structure of marshes created by planting native bioengineering grasses Spartina alterniflora and Spartina densiflora in a micromareal estuary, through the quantification of the autogenic process and its relationship with the availability of N and P, estuarine environmental heterogeneity and regional forcing . In Chapter I, two experiments were carried out in an agricultural greenhouse to evaluate the effects of planting density and availability of nutrients N and P on the development of S. alterniflora and S. densiflora grasses. Under high availability of nutrients, plants of S. alterniflora had their leaf production stimulated and produced twice the number of stems than unfertilized plants.Initial densities of 400 or more m-2 stems resulted in excessive vertical elongation of the stems of S. alterniflora (about 100 cm in height). Fertilization with a 2N: 1P ratio resulted in better tillering of both species. Chapter II describes the similarities and differences in the structural characteristics of marshes created by planting bioengineering grasses and natural reference marshes, sampled in 4 locations in the Lagoa do Patos estuary (RS) during the summer-autumn 2012. Twelve species from 8 families angiosperms were found in 597 squares (0.5 X 0.5 m) observed. All salt marshes revealed the presence of S. alterniflora on the floors between low tides and mature marshes exhibited 3 to 7 species. Grouping and Correspondence Analysis, according to the visual coverage correlation matrix,distinguished the mature everglades from most of the created and young everglades. The sampled salt marshes were distributed between -0.28 m and +0.91 m NML (average pond level) and were subject to flooding frequencies from 8.0 to 83.8% of the time. Mature marshes were higher in the tides than local created and young marshes, as well as having higher levels of organic matter (up to 26.43%) and clays (up to 50%) in the sediment, in addition to lower levels of phosphate in the water. interstitial. The results suggest strong organic uplift and high phosphorus consumption due to the large plant biomass present in mature marshes. The low position in the intertidal of the created marshes hinders the development of their soils (low levels of fines and organic matter) and results in positive redoxs, due to the beat of waves.Organic uplift (4-5% organic matter), low redoxs (up to -428 mV) and high levels of N and P distinguish the most continental part of young marshes in the eutrophic Saco da Mangueira cove from their low areas in the tides and the created marshes . Larger aerial biomass formations on these high floors of young marshes are made possible by the greater availability of P in reduced conditions. Chapter III evaluates the roles of autogenic and allogeneic processes in the development of a salt marsh created by planting S. alterniflora over 3 years (2012-2015). The development of the salt marsh has progressed in an autogenic manner, despite sequential interannual oscillations of low flow and excessive flow associated, respectively, with events of moderate intensity in La Niña and El Niño. The planting blocks were flooded 32,9-51.5% of the time for waters with an average salinity of 23-26 in 2012, for 61.9-69.5% of the time for water oligohalins (<5) in 2014. In 2012, the sandy sediment of the intertidal plane it had less than 7% fine, accumulating until 2015 high average contents of clays (21.9 ± 10.9%) and organic matter (14.7 ± 4.6%). The plant biomass in the reared pond grew to maximum values ​​in the second year after planting (respectively, aerial = 1325-1364 g / m2 and underground = 799-810 g / m2). The concentrations of interstitial nutrients in the planting blocks reflected the vigor of the autogenic alteration, with a reduction in the levels of phosphate in the interstitial water possibly related to consumption by vegetation, aeration of the rhizosphere by the plants and / or organic uplift of the tide, with a consequent increase in the potential redox.The autogenic role in the structural development of the created salt marsh appears to be supported by the high availability of nutrients in the superficial and interstitial waters of Saco da Mangueira.