Changes in salt marsh vegetation were analyzed over a two-year period (November 1984-November 1986 following the construction of a dike in the southwest corner of Punta Banda Estuary, Baja California, Mrxico.Changes included: increased interstitial soil salinity, reduced soil moisture, increased mortality of Spartina foliosa and decreased middle marsh species diversity due to the elimination of annual and short-lived species.The sea-side edge of the middle marsh shifted its boundary to a lower elevation. By the end of 1986, dominant species were Salicornia virginica, Batis maritima and Frankenia grandifolia. By October 1988, only a few patches of S. foliosa persisted at the water edge, and it appeared that the community was not yet stable. The potential future of the estuary is evaluated.
To describe the annual cycle of Sargassum horneri in Mexican waters, we selected two sites differing in their degree of wave exposure and sediment type: Rincón de Ballenas (RB), and Rancho Packard (RP). From June 2009 to April 2010 we followed the seasonal changes in S. horneri density and biomass along two intertidal transects per site. The effects of this non-indigenous species on the local macroalgae community were assessed by comparing their species composition, density, biomass, species richness, and diversity index in quadrats with and without S. horneri. There were significant differences in S. horneri density and biomass between sites (P < 0.001). At RB the invasive alga density average was 2 ± 0.94 individual m −2 , with a mean biomass of 4 ± 0.95 g DW m −2. At RP, S. horneri density average was 10 ± 0.96 individual m −2 , and mean biomass of 102 ± 0.97 g DW m −2. At RB, the invasive alga promoted a significant reduction in the four selected structural variables, and the corticated macrophytes and the foliose functional forms were severely reduced. At RP, there were only marginally significant effects (P = 0.06) of S. horneri presence on the local macroalgae community, and higher density, biomass, and diversity values were found when S. horneri was present. Most of the functional forms were found, even if the invasive alga was present. At both locations, the highest biomass corresponded to the articulated calcareous functional form. These contrasting results could be due to the fact that the native macroalgae community has already been altered by the early invasion of S. muticum, with the most resilient species and functional forms remaining in place. One of the most important changes we noticed is the severe reduction of the canopy forming species at both sites.
Disturbance is an important factor influencing plant species composition and diversity. We addressed changes in plant composition and soil characteristics in Estero de Punta Banda, Baja California, Mexico following 22 years of disturbance by tidal exclusion. Currently, sediments in the non-tidal site are dry, 26±1% moisture, and hypersaline, 143±12; while those at the tidal marsh are wet, 36.2±1% moisture, with 40.3±2.6 salinity. The nontidal site has lost seven species including annuals, shortand long-lived perennials, ephemerals, and parasites. Current dominants are the perennials Batis maritima and Sarcocornia pacifica. Average species richness at the nontidal site is 4.4±0.32 vs.10±0.18 species per square meter at the tidal site. Average species diversity index is lower at the diked area. The general biodiversity loss that results from tidal exclusion in arid estuaries, contrasts with the species-rich communities that develop in diked humid-climate estuaries.
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