The goal of this study was to analyze microbial mats and biofilms from the lower supratidal area of the Bahía Blanca estuary (Argentina), and explore their relationship with sediments and other physical forcings. Thirteen monthly sediment samples (uppermost 10 mm) were taken and their composition and abundance in microorganisms was determined by microscopy. Physical parameters (solar radiation and sediment temperature at -5 cm) were recorded with a frequency of 5 minutes by a coastal environmental monitoring station. Additionally, sediment grain size and moisture content were determined for distinct layers in the uppermost20 mm, and the rate of inundation of the supratidal area was estimated from tidal gauge measurements. There were significant seasonal differences in the biomass of the microphytobenthic groups considered (filamentous cyanobacteria and epipelic diatoms), with the former consistently making up >70% of the total biomass. The relationships between microphytobenthos and sediment temperature and solar radiation fitted to linear regressions, and consistently showed an inverse relationship between microphytobenthic abundance and either one of the physical parameters. The granulometric analysis revealed a unimodal composition of muddy sediments, which were vertically and spatially homogeneous; additionally, there were significant seasonal differences in water content loss with drying conditions prevailing in the summer. Several Microbially-Induced Sedimentary Structures (MISS) were identified in the supratidal zone such as shrinkage cracks, erosional pockets, gas domes, photosynthetic domes, mat chips and sieve-like surfaces. In contrast to studies from analogous environments in the Northern Hemisphere, we found reduced microphytobenthic biomass in summer, which were explained by increased evaporation/desiccation rates as a consequence of increased radiation, despite frequent tidal inundation. In conclusion, the observed density shifts in the benthic microbial communities are attributable to physical forcings dependent upon seasonal variations in interplaying factors such as sediment temperature, solar radiation and tidal inundation.
Populations of photo-autotrophic microorganisms, collectively known as microphytobenthos, often develop in intertidal and lower supratidal zones, becoming the most important primary producers [1,2]. The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow within the upper several millimeters of illuminated sediments [
Microphytobenthic organisms may produce macroscopically-recognizable microbial mats dominated by cyanobacteria, or visible biofilms of epipelic diatoms. The community structure is determined by physical parameters such as temperature, light, physical resuspension, and in turn, the biogeochemical activity of the microorganisms determines some sedimentary properties. For example, these organisms secrete large amounts of Extracellular Polymeric Substances (EPS), which are known to aid in their vertical migration [5,6] in response to light and tidal conditions [
The capacity of microphytobenthos to biostabilize sediments in low-energy environments [
The goal of this study was to explore the relationship between microphytobenthos, sediments and physical factors in a siliciclastic tidal flat colonized by microbial mats in the Bahía Blanca estuary. We relate the seasonal changes in atmospheric and physical variables to the biotic components of microbial mats, with the aim of characterizing the resulting sedimentary structures.
Puerto Rosales (38˚55'S; 62˚03'W) is located on the northern margin of the central zone of the Bahía Blanca estuary, Buenos Aires Province, Argentina (
A semiarid temperate climate characterizes the area, with a mean annual air temperature of 15.6˚C (mean temperatures range from 22.7˚C in January to 8.1˚C in July). Surface seawater mean annual temperature at Puerto Rosales is 14.1˚C. On average, cumulative solar radiation in a cloudless day is 28 MJ·m−2 in summer and 11 MJ·m−2 in winter [
In Puerto Rosales, extensive tidal flats (~ 1000 m wide) with low slopes (~0.4˚ gradient) are composed of sandy to muddy siliciclastic sediments. Siliciclastic grains predominantly consist of quartz with minor amounts of feldspars. The supratidal area is flooded by seawater, reaching ~ 10 cm depths during spring high tides. Local winds from SW to NE sectors generate waves with short wavelengths and <6 s periods. The significant wave height in Puerto Rosales is 0.3 m [
rate and favoring the colonization of benthic microbial communities that form biofilms and microbial mats [
Thirteen sampling events of the microphytobenthos were carried out (dates specified as Julian days on
Physical parameters (i.e., solar radiation and sediment temperature) were recorded by means of a coastal environmental monitoring station (EMAC after its initials in Spanish; http://emac.criba.edu.ar/). The EMAC station, placed in situ at Puerto Rosales, is equipped with an APOGEE SP-110 radiation sensor (W·m−2) located 3 m above the sediment surface. The temperature sensors (developed by A. Vitale) with a recording range from −15˚C to 60˚C and 0.1˚C resolution (error + 0.1˚C) were placed 5, 15 and 30 cm into the sediment. All sensors have a measuring frequency of 5 minutes.
A discrete number of samples (sediment cores) were taken using sawn-off 50-ml medical syringes, and separated into three layers (layer thickness ranging from 4 to 10 mm). Sediment grain size was determined for each layer with a laser diffraction particle analyzer Malvern Mastersizer 2000, for particles in the 0.2 - 2000 μm range (i.e. colloids to sand). Organic matter was oxidized prior to the analysis by adding H2O2 with heating and stirring. Additionally, sediment samples (n = 3) were obtained to determine moisture content in June and December, in order to evaluate the extent of desiccation on the tidal flat during the Austral winter and summer. Sediment moisture was calculated from weight differences before and after drying samples at 60˚C to a constant weight for 96 h [
Seasonal differences in diatom and cyanobacteria biomass were tested by means of a two-factor ANOVA, with replication [
The microphytobenthic community integrating biofilms and microbial mats (10 mm in depth) consisted in unicellular (epipelic diatoms) and filamentous microalgae (cyanobacteria). The smaller pennate diatoms (<40 μm) included the genera Diploneis, Nitzschia and Navicula, while the larger-sized representatives included species of the latter two genera, and also the species Gyrosigma spencerii and Cylindrotheca closterium. Centric diatoms included the genera Thalassiosira, Coscinodiscus and Melosira. All cyanobacteria found in the sediments were non-heterocystous, with Microcoleus chthonoplastes being the dominant species, and the genera Oscillatoria and Arthrospira being also present.
There were significant differences in the biomass (expressed as cell biovolume) of the two groups of microphytobenthos considered [two-factor ANOVA, F (1, 51) = 21.79; p < 0.001] (
The relationship between diatom and cyanobacteria biomass, and sediment temperature and solar radiation (SR) was analyzed by means of linear regressions. For both parameters the trends were similar, with inverse relationships between the biomass of either cyanobacteria or diatoms and the environmental parameter. The relationship between cyanobacteria biomass and sediment temperature was fitted to a linear regression (ANOVA, F(1, 12) = 4.96, p < 0.05; r2 = 0.31) (
Averaged sediment temperature (measured at −5 cm) ranged from 5.3˚C on 6/28/2011 (Austral winter; coinciding with the maximum biomass registered for cyanobacteria, see
The granulometric analysis revealed a unimodal composition of the muddy sediments at the study site, which was vertically and spatially homogeneous (
Water content measurements during emersion in winter and summer conditions resulted in greater values on winter, with minor differences of water content (<1%)
lost on the upper layers, while summer evidenced overall drying conditions (
Several MISS were identified in the supratidal zone of the tidal flat, among which the most conspicuous are: shrinkage cracks, erosional pockets, gas domes, photosynthetic domes, mat chips and sieve-like surfaces.
Shrinkage cracks constitute mat-destruction structures formed due to subaerial exposure and desiccation of the mats, typical of supratidal areas [
irregular pattern of the cracks network defines polygons with a variety of sizes, ranging from about 3 to 30 cm. Sometimes, the edges of the polygons become rounded and curled up, widening the open cracks from 1 to 10 cm (
Erosional pockets are derived from the mechanical destruction of the biostabilized sediment surface [27,28].
These structures evolve from local destruction of the microbial mat that covered the surface sediment during high energy conditions, generating irregular-shaped depressions [
Gas domes occur when the gases trapped beneath the relatively impermeable microbial film accumulate, increasing the pressure and pushing the mat upward, producing a bulge and a hollow cavern underneath [
Photosynthetic domes are small domes formed by deformation of the elastic surface layer due to accumulation of oxygen released as a product of photosynthesis by underlying cyanobacteria [
Sieve-like surfaces are the product of numerous tiny surface pits created by the impressions of biostabilized photosynthetic gas bubbles formed on the mat surface [
Microbial mat chips are sedimentary features related to the physical mat-destruction that comprise eroded mat fragments, which reflect reworking and transport (
Microbial mats that develop in the supratidal zone of estuarine sediments have been termed “epibenthic mats” [
The Bahía Blanca estuary, is a typical temperate estuary (sediment temperature ranges between 5.3˚C and 27˚C and SR between 72 to 323 W·m-2), that shows no evidence of extreme physical forcings, that might hinder the development of microbial mats and biofilms throughout the year in contrast to what has been reported for estuarine systems in the Northern Hemisphere. However, the inverse relationships between microphytobenthic biomass and temperature and SR (
It is worth mentioning that an annual survey of planktonic production [
One plausible explanation for the reduced microphytobenthic biomass in summer lies in the increase in evaporation/desiccation rates in the tidal flat, as a consequence of increased radiation. In our study, average radiation roughly varied 4.5-fold between winter and the onset of summer, likely creating hypersaline conditions in pore water and making the microorganisms osmotically-stressed, resulting in slowed physiological processes and little growth [
evaporation, and this is a cumulativeprocess, with sediments consolidating as water content becomes progresssively lower [
Sediment grain size plays a pivotal role in the distribution and abundance of microphytobenthos [
The study area is characterized by the abundance of cohesive silts in the topmost layers of sediment, which retain high moisture content. The characteristic laminated pattern is a product of the abundance of cyanobacteria and the spatial arrangement of grain sediments produced by these microorganisms [
The supratidal flats at Puerto Rosales are composed of cohesive silty sediments that retain moisture. These sediments and the microphytobenthos are continuously affected by sea-air interactive processes. No previous studies on physical-biological interactions were performed for this area. We report significant variations in the biomass of cyanobacteria and epipelic diatoms throughout the year, with maxima in winter, which is in stark contrast to the pattern reported for analogous systems in the Northern Hemisphere. These density shifts are attributable to physical forcings dependent upon seasonal variations in interplaying factors such as sediment temperature, solar radiation and tidal inundation.
This study was funded by Agencia Nacional de Promoción Científica y Tecnológica (PICT 374/07) and Secretaría General de Ciencia y Tecnología—UNS (PGI 24/ ZH20); the former institution supported CNB in the form of a fellowship. JP was partially supported by a Postdoctoral Fellowship from CONICET.