SUPPLEMENTARY MATERIALS
Storrs Lake reaches a maximum depth of two meters during the wet season (MayNovember) due to rainfall and groundwater discharge (Mann & Nelson, 1989). During
the dry season (December-April), evaporation reduces the water level. Some tidal influx
and inflow of seawater occurs, but these effects are negligible, and daily tidal fluctuation
has not been observed (Mann & Nelson, 1989). Storrs Lake salinity levels fluctuate
seasonally from 38 PSU during the wet season to 93 PSU during the dry season. The
water is red-brown and highly turbid from sediment and organic matter in suspension.
The temperature of the water can reach 40ºC during summer, and the pH of the water
column ranges from 8.1 to 8.6. The lake water has an average carbonate alkalinity of ca.
800 mg/L HCO3- and is supersaturated with respect to calcite and aragonite (Table 1).
Northern Lake Sector (Fig. 1)
Extensive carbonate deposits developed in the northern sector during the last 3,000 years
(Hattin, 1982). Previous studies of transects in this sector showed a succession from
laminated carbonate crusts near the shoreline, to bulbous crusts, followed by larger
mushroom and club-shaped heads in the deeper regions toward the center of the lake
(Fig. 2a; Neumann et al., 1989; Mann & Nelson, 1989; Paull et al., 1992). The
heterocystous filamentous cyanobacterium Scytonema sp. is responsible for the formation
of the bulbous crust (Pentecost, 1989).
14
C dates performed on the mushroom-shaped
heads show an age of 2,310 ±70 years at the base (Paull et al., 1992). ∂18O and ∂13C
isotopic data suggest a complex history of carbonate precipitation and dissolution that
makes basin-wide correlations very difficult (Paull et al., 1992). Microbial
photosynthesis and N2-fixation are currently not very active in the northern sector
(Bebout, 1992, and this study).
Southern Lake Sector (fig. 1)
The southern sector was the last basin to be closed off from the open ocean (Zabielski,
1991).
The present study focuses on two transects (Fig. lD). The first transect (Fig. 2B1) is in the
shallow part of the lake (maximum depth ca. 40 cm) and shows high temperature
fluctuations (25°-45°C). The salinity ranges from 35 to 90 PSU and the pH averages at
8.5. This transect is characterized by small laminated calcareous knobs built by active
microbial mats but lack the presence of large stromatolitic heads. From the edge toward
the center of the lake, the following two zones can be distinguished along the transect
(Fig. 2B1).
(1) Zone I is characterized by carbonate sand that is trapped and bound by filamentous
cyanobacteria and ‘cemented’ in the upper 5 mm to form a cohesive crust (Fig. 3C). This
crust has a red-brown to green coloration in the two millimeters immediately underneath
the dark surface.
(2) Zone Ia is comprised of a thick microbial mat dominated by filamentous
(Phormidium-like) and coccoid (Entophysallis sp. and Gloeocapsa sp.) cyanobacteria.
The microbial mat can be more or less cohesive (less cohesive in the wet season) and
shows the classical sequence of depth horizons (from top to bottom): a brownish layer
consisting of empty sheaths embedded in extracellular polymeric substances (EPS) that
includes photopigments (e.g., gloeocapsin, scytonemin), a green layer of oxygenic
phototrophs, a pink layer of anoxygenic phototrophic purple sulfur bacteria, and a black
FeS layer dominated by sulfate-reducing bacteria. The presence of variable amounts of
translucent greenish to reddish ‘slime’ (Fig. 3a,b), which has been referred to as
‘thrombolitic pie mounds’ (Mann and Nelson, 1989), ‘ectoplasm’ (McNeese, 1988), or
‘jelly bombs’ (Paerl, pers. com.) is unique to Zone Ia mats. The jelly bombs consist of the
filamentous cyanobacteria Phormidium henersonii (Mann & Nelson, 1989) and various
coccoid cyanobacteria embedded in a large amount of EOM. Jelly bombs initially form as
small, flat, disc-like shapes within the microbial mat (Fig. 3c) and develop further into
large ‘balloon-like’ structures made up of translucent organic matter (i.e., the actual jelly
bombs).
When the water depth increases along the transect, the microbial mat progressively shows
a more flocculent texture (Fig. 3D), referred to as ‘ooze’ (Mann and Nelson, 1989) that
covers the entire deeper section of the lake. The ooze appears dark gray in color, but
more careful inspection shows that the upper millimeters are made up of a pigmented
brown layer that is overlain by green and red layers (Fig. 3D). Large jelly bombs are
present in the ooze.
(3) Zone II shows patches of calcifying, finely laminated knobs (Fig. 3E), which are
formed by a Scytonema sp.-dominated microbial mat. The knobs often have conical
shapes (Fig. 3F), and are surrounded by ooze. Knobs are approximately 5 cm high, and
are grouped in patches.
Transect B2 (Fig. 2 B2) is characterized by a steeper increase in water depth. The
resulting increased accommodation space allows for Scytonema knobs with a more
complex morphology to develop. Due to the steeper bottom topography: (1) Zone I
covers only ca. 1-2 m and, similar to transect B1, is characterized by a cohesive sediment
crust. Zone Ia (with jelly bombs) is absent in this transect, (2) Zone II displays patches of
sub-calcified Scytonema knobs comparable to Zone II of transect B1, (3) In Zone III, the
increased accommodation space allows for the knobs to develop more complex
morphology and a greater degree of calcification. These microbialites have an irregular
macrostructure, and (4) Zone IV is dominated by ooze overlaying a black crust. The inner
part of the ooze shows a dark gray to black color typical of a reducing environment.