Foraminiferal zonation from a subtropical mangrove in Bertioga Channel (São Paulo, SP, Brazil)

Abstract

This study focus on foraminiferal distribution is from two intertidal mangrove ecosystems is located in the Bertioga Channel (São Paulo, Brazil) and its correlation to environmental gradients. Despite of its vicinity to Sao Paulo and Santos cities, the most populated regions in the country, Bertioga Channel is still partially pristine showing that the balance between fresh water and marine waters still provides natural estuarine variability, which are responsible for the foraminiferal zonation of agglutinated species. The zonation mainly relates to tidal influence, where Haphlophragmoides wilberti inhabits less saline environments than Arenoparrella mexicana. The mangrove located closer to the northern mouth of the Bertioga Channel is a more saline environment with higher numbers of dead calcareous species, evidencing greater tidal influence than the mangrove located near the southern mouth. Distribution of Bolivina striatula and Pararotalia sp, demonstrate a greater marine influence in the Crumaú River than in the Trindade River. Here we show what and why the differences between foraminiferal associations from two mangroves and surroundings are taking place in the recent environment establishing a baseline of understanding paleoclimate alterations and sea level changes.

Introduction

Estuaries and mangroves, transition regions between sea and river, are one of the most productive environments on the planet, where natural dynamics is of fundamental importance for the balance and sustainability of coastal ecosystems. These environments are highly variable in physicochemical conditions, within the natural features that characterize an estuary. Geomorphology, continental drainage, and tides are the dominant variables that determine the distribution of salinity and circulation (Hansen and Rattray Jr., 1966) in these environments. These transition regions represent a challenge to the study of the ecosystem; and the evaluation studies depend on the survey of the greatest possible number of abiotic information. A simple biological indicator is sensitive enough to react rapidly to changes and is capable of synthesizing and monitoring the general environmental characteristics. With foraminifera as indicators, it is often possible to detect and map different characteristics of the estuarine circulation and the salinity distribution (Scott et al., 1980), and its documented sensitivity suggests that these organisms indicate recent environmental changes in coastal areas from both natural and anthropogenic actions (Hayward and Hollis, 1994, Williams, 1995).

The community structure and its characteristics related to the abundance of individuals, specific composition of the associations, diversity and dominance are useful for the recognition of different environments. The use of foraminifera as bio indicators, in turn, is not only a practical and efficient tool, but is also relatively inexpensive and of great potential in generating information about the environmental dynamics.

Compared with other regions of the world, Brazil has the largest number of estuaries per kilometer of coastline, but with low incentive from government and other funding agencies, the number of environmental specialists working in Brazilian estuaries is small. In contrast to mid-latitude temperate environments there have been relatively few studies of intertidal foraminifera and their relationship to Holocene sea level tropical coastal environments. The latest studies include New Zealand (Hayward et al., 1999, Hayward et al., 2004), west coast of Africa, New Caledonia and northern Australia (Debenay et al., 2000), Northern Australia (Haslett, 2001), French Guiana (Debenay et al., 2002), (Bermuda), Javaux (1999) and Javaux and Scott (2003), Great Barrier Reef coastline, Australia (Horton et al., 2003), and Indonesia (Horton et al., 2005). Our study will attempt to fill in the previous gaps in the knowledge of the marine micropaleontology from subtropical mangroves.

Even with the proximity to the city of São Paulo, the Bertioga Channel ecological state lacks information about its ecosystems. It is known however, that most of this ecosystem is pristine, which emphasizes the importance of diagnostic studies that subsidize information regarding its natural state in the face of the growing development of the Bertioga region and its surroundings.

We hypothesize that the difference in the distribution of foraminiferal from two mangroves occurs because of their proximity/distance to freshwater sources and marine intrusion by tides. We intend to show how these differences are taking place and that the zonation of Foraminiferal species between marine, mixohaline, brackish and continental faunas are gradational and very specific. Faunal response is accurate and we believe that detailed studies on the estuarine/salinity balance of recent environment and foraminiferal zonation establish new findings that will provide guidance on the use of lesser-known South American mangrove environments and should be of interest in paleoclimate studies, detecting oceanic to freshwater patterns, and in reconstructing sea level from fossil cores.