Hrvatski geološki institut

Ophiolite exposures in NW Croatia have been attributed to the Western Vardar Ophiolitic Unit and interpreted as derived from the Meliata-Maliac-Vardar branch of the Neotethys. Blocks within the ophiolitic mélange on Mt. Ivanščica were investigated for petrological and geochemical characteristics of effusive rocks and radiolarian dating of associated pelagic sedimentary rocks. Analysed effusive basic rocks represent chemographically uniform sub-alkaline high-Ti massive tholeiitic basalts characterized by an enriched composition typical of E-MORB. These basalts are compatible with approximately 9 to 11% of partial melting of an enriched mantle source transitional between primitive and depleted MORB-type mantle and are formed in the non-subduction geotectonic setting of E-MORB-type. This reflects an initial succession of oceanic protocrust formation and the onset of ocean spreading. Radiolarians from chert and shale succession associated with basalts indicate a Late Anisian to Early Ladinian age of the initial ocean floor spreading, which continued into the Langobardian. Obtained data are correlative with reported blocks interpreted as remnants of the Triassic Neotethys crust from the ophiolitic mélange of the Western Vardar Ophiolitic Unit and further reaffirm common origin from a single ocean basin located east of the Adria microplate. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

A complex and chaotic assemblage of the north Croatian inselbergs, considered as a real geologic conundrum, is marked by omnipresent ophiolite mélange. This is a vestige of the unbroken formation of the lithosphere in the northwestern segment of Neotethys during the Mesozoic, spanning from the late Anisian to the late Tithonian. In this contribution, we present detailed mineralogical, petrological, geochemical, and isotopic data on the mélange collected from 1991 to 2023. These data include the entire normal ophiolite sequence, from mantle tectonites and cumulate ultramafic rocks through cumulate and isotropic gabbro and sheeted dyke complex to massive and pillow lavas with interbedded radiolarian cherts. We found that the continuous development of oceanic lithosphere during the Mesozoic has been evidenced by seven geochemical groups of ophiolitic rocks, each representing a distinct basalt-gabbro suite. Based on their geochemical and isotopic characteristics, a sequence of petrogenetic processes and tectono-magmatic events has been reconstructed. Proposed geodynamic models which shed light on the Mesozoic evolution of the northwestern segment of Neotethys are consistent with the current geodynamic understanding of the broader Mediterranean region. The similarities in the tectono-magmatic and geodynamic history of NW Neotethyan ophiolites and ophiolites from the Dinaridic-Albanide-Hellenide belts suggest that they evolved together, likely within a single branch of the Neotethys Ocean during Triassic to Jurassic time. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Upper Triassic to Lower Cretaceous stratigraphic successions deposited on the passive continental margin of Adria microplate facing the Neotethys Ocean were investigated on Ivanščica Mt. (NW Croatia). This area represents a transitional area between the Alps and the Dinarides. After a short Middle Triassic pelagic episode related to the rifting of the Neotethys, progradation of the carbonate platform over newly formed grabens resulted in formation of stable shallow-water depositional environment during the Late Triassic. Tectonically induced subsidence pulse at the Triassic/Jurassic boundary affected SE part of the investigated area resulting in the deposition of pelagic limestones, shale, marls, and calcarenites above Upper Triassic platform carbonates. At the same time, NW part of the investigated area was still characterized by shallow-water sedimentation until the Pliensbachian. This second subsidence phase was manifested by formation of neptunian dykes within Lower Jurassic platform limestones and deposition of the Middle Jurassic pelagic limestones. These two Early Jurassic subsidence events are likely related to the rifting of the Alpine Tethys. Radiolarian cherts were deposited in both areas in the late Middle and Late Jurassic. Radiolarians from the SW Ivanščica Mt. indicate late Bathonian to Early Tithonian age of the radiolarian cherts. Uppermost Jurassic to Lower Cretaceous Aptychus limestone is found across the entire investigated area. These limestones were deposited from the Late Tithonian to the Valanginian when synorogenic mixed carbonate–siliciclastic turbidites started to fill the basin indicating a prominent compressional tectonic phase widely recorded in the Alps and the Dinarides.

Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny.

The sustainable utilization of geothermal energy mostly depends on the characteristics of the geothermal resource from which it is extracted. Among others, detailed geological modeling is a key factor for estimating the potential of a geothermal resource. This research focuses on the modeling and reconstruction of the geological setting of the Daruvar thermal spring area using geophysical techniques. An integrated geophysical approach based on electrical resistivity tomography (ERT) and both active and passive seismic (MASW and HVSR) methods was used. Based on ERT results and the stratigraphic logs of the wells in Daruvar, three resistivity layers/geological units were identified. The deepest layer with resistivity < 150 Ωm is the Triassic carbonate that constitutes the thermal aquifer. Sharp lateral variations in the resistivity distributions within the bedrock were interpreted as fault damage zones saturated with thermal waters. Integrating the results of the seismic methods, the thickness of the first seismic layer that corresponds to the Quaternary cover was estimated from 5 to 20 m. Here, results of the geophysical investigations were combined into a 3D geological model highlighting the occurrence of subvertical N-S and E-W trending faults in the Daruvar spring area. The N-S-trending fault was interpreted as a fault plane parallel to the regionally mapped Daruvar fault. This fault juxtaposes the Triassic carbonate complex of the thermal aquifer with a Neogene sedimentary sequence of significantly lower permeability. Neogene–Quaternary tectonic activity further increased the fracturing and the permeability field in the Daruvar spring area, as proven by the smaller scale E-W faults and the well logs. This fracture network permits a quick upwelling of thermal fluids resulting in thermal springs with temperatures up to 50 °C. This work proves that the construction of a detailed geological model is crucial for assessing the reservoir and fault geometries in thermal systems hosted in fractured carbonate rocks.

Global warming has been monitored for many years. The increase in air temperature and changes in the distribution and frequency of high temperatures are recorded continually. Lakes are one of the important water resources for aquatic ecosystems and water supply, which are significantly affected by global warming. The increase in lake water temperature increases the evaporation from the free lake surface, lowering the lake level, and changes the water quality. In the last few decades, analysis of changes in lake water temperature has been increasing. In situ measurements of water temperature in Vrana Lake on Cres island (Croatia), the largest freshwater lake on the Mediterranean islands, were analysed over 43 years. The results showed that the mean annual lake surface water temperature (LSWT) increased by 0.47 °C decade-1 (p

Footprints of human activities identified in the sedimentary sequence of submerged historical saltpans can reveal the history of the site and can indicate the relative sea level during its operational period. Saltpans are man-made constructions used continuously for salt production in the Mediterranean at least for the last 2000 years. The east Adriatic coast contains many such submerged remains, preserved and well-dated by historical archives. Sedimentological, microfossil and geochemical analyses of the sediments from cores drilled in the saltwork area at Brbinj, Dugi Otok, Croatia, enable the reconstruction of various past environmental conditions. The current study aims to: a) identify the anthropogenic unit in the sedimentary sequence deposited over time, b) determine its age, and c) use it as past sea-level limiting points. Basal units made of terra rossa soil materials were identified in the sedimentary records. These layers are located -120 ±7 cm below mean sea level next to the separation wall and -125 ±7 cm and -135 ±7 cm, respectively, in the inner pools, most likely representing a man-made pavement. The terra rossa layer is overlaid by a unit rich in faunal remains dominated by euryhaline foraminifera and ostracod species such as Ammonia veneta and Cyprideis torosa, representing the saltworks unit. The flooding of the saltpans by the rising sea is manifested by the deposition of an upper sedimentary unit dominated by remains of marine species. The base and the top of the saltwork unit are dated by Optically Stimulated Luminescence to 1040±50 CE and to 1390±30 CE, respectively. The study presents a new approach for obtaining footprints of human activities in ancient, submerged saltpans, by identifying and dating the indicative anthropogenic layers and using these for the reconstruction of paleo sea-level. The described method can be applied all around the Mediterranean.

During the pumping of wells, the groundwater level drawdown, as measured in the pumped well, is increased by non-linear losses caused by the water flow velocity through the well screens. This undermines the adequacy of the direct use of the measured drawdown data in the well for the purpose of the realistic identification of the effective well radius and aquifer parameters. This anomaly is avoided by reshaping the drawdown function into a function of the specific drawdown sw/Q of the pumped well. This reshaping simplifies the exclusion of non-linear losses from the sequence of measured data of the water level in the well at the position of the effective radius of the pumped well. Combining the data of linear losses and the respective pumping rate of the pumped well, a function of the specific drawdown of the radial flow sw/Q was formed. This function describes the aquifer parameter relations during the respective test pumping. A consistent sequence of the function of the specific drawdown sw/Q of the pumped well reveals the actual value of the coefficient of nonlinear losses. Moreover, the specific drawdown function enables the reliable estimation of aquifer transmissivity using only the pumped well drawdown data.

Over the last decades, the quality and quantity of the Mediterranean freshwater resources have significantly deteriorated due to climate change, unsustainable utilization, user conflicts, and seawater intrusions. On the small and remote island of Vis, where similar issues prevail, the need for alternative water management solutions has yielded managed aquifer recharge (MAR) as a promising option for increasing the safety and resilience of the local and autonomous water supply. By performing a cost–benefit analysis (CBA) to evaluate the feasibility of the deployment of an infiltration pond method in the Korita well field, the results evidenced a positive financial performance and sustainability of the proposed MAR solution. In addition, the overall economic benefits of the project, quantified through the willingness-to-pay method, significantly exceeded its costs, as evidenced by the high benefit/cost ratio of 2.83. The most significant uncertainty related to the infiltration pond method is represented by the high sensitivity to changes in the applied hydrological assumptions (i.e., the evaporation coefficient and number of annual infiltration pond recharges). This study aims to contribute to the understanding of interrelated socio-economic factors of MAR projects in karst aquifers, and represents the first of its kind in Croatia.

Hot spring biofilms are stable, highly complex microbial structures. They form at dynamic redox and light gradients and are composed of microorganisms adapted to the extreme temperatures and fluctuating geochemical conditions of geothermal environments. In Croatia, a large number of poorly investigated geothermal springs host biofilm communities. Here, we investigated the microbial community composition of biofilms collected over several seasons at 12 geothermal springs and wells. We found biofilm microbial communities to be temporally stable and highly dominated by Cyanobacteria in all but one high-temperature sampling site (Bizovac well). Of the physiochemical parameters recorded, temperature had the strongest influence on biofilm microbial community composition. Besides Cyanobacteria, the biofilms were mainly inhabited by Chloroflexota, Gammaproteobacteria, and Bacteroidota. In a series of incubations with Cyanobacteria-dominated biofilms from Tuhelj spring and Chloroflexota- and Pseudomonadota-dominated biofilms from Bizovac well, we stimulated either chemoorganotrophic or chemolithotrophic community members, to determine the fraction of microorganisms dependent on organic carbon (in situ predominantly produced via photosynthesis) versus energy derived from geochemical redox gradients (here simulated by addition of thiosulfate). We found surprisingly similar levels of activity in response to all substrates in these two distinct biofilm communities, and observed microbial community composition and hot spring geochemistry to be poor predictors of microbial activity in the study systems.

The upper part of the Krka River estuary and Prokljan Lake are a specific example of a well-stratified estuarine environment in a submerged river canyon. Here, we reconstructed the geomorphological evolution of the area and classified the data gathered in the study, integrating multibeam echosounder data, backscatter echosounder data, side-scan sonar morpho-bathymetric surveys, and acoustic sub-bottom profiling, with the addition of ground-truthing and sediment analyses. This led to the successful classification of the bottom sediments using the object-based image analysis method. Additional inputs to the multibeam echosounder data improved the segmentation of the seafloor classification, geology, and morphology of the surveyed area. This study uncovered and precisely defined distinct geomorphological features, specifically submerged tufa barriers and carbonate mounds active during the Holocene warm periods, analogous to recent tufa barriers that still exist and grow in the upstream part of the Krka River. Fine-grained sediments, classified as estuarine sediments, hold more organic carbon than coarse-grained sediments sampled on barriers. A good correlation of organic carbon with silt sediments allowed the construction of a prediction map for marine sedimentary carbon in this estuarine/lake environment using multibeam echosounder data. Our findings highlight the importance of additional inputs to multibeam echosounder data to achieve the most accurate results.

Tracer testing is the only method in karst hydrogeology that can definitively determine whether a particular site belongs to a watershed of a particular karst spring. Therefore, it is an essential technique for delineating groundwater basins in karst areas. The availability of tracer test results is often limited due to the complicated and relatively expensive application of this approach, especially for large regional watersheds. The Croatian part of the Dinaric karst region extends for several hundred kilometers along the Adriatic coast and consists almost entirely of highly karstified carbonate rocks. The groundwater basins in these areas almost never match the surface morphology of the terrain. In practice, all available results of previous surveys are often used to define watersheds, regardless of the methodology and age of their implementation. This is also true for the earlier delineations of the Gacka River watershed, a regional karst basin in the Croatian Dinaric karst. However, tracer testing methods, especially the accuracy of tracer determination and monitoring, have improved significantly during this time. In order to assess the reliability of past tracing results in this significant karst basin, we reviewed reports of previous tracer tests. More recent tests, in particular the most recent multitracer injection test with continuous tracer detection on the major springs, produced high-quality data that allowed us to assess the reliability of the findings from prior research. A number of large karst springs with partially overlapping subcatchments feed the Gacka River. After discarding unreliable tracing data, we reevaluated the subcatchments of the main springs as well as the characteristics of the regional groundwater flow patterns throughout the basin, which is particularly important for water quality protection measures of the springs. The Gacka River basin is used as a case study to emphasize the importance of thoroughly assessing the reliability of previous tracing data before using them in regional analyses.

Paleoecologic (paleoclimatologic) and biostratigraphic studies of pelagic and deep-water deposits rely on the identification of planktonic foraminifera. Here we report and compare the results of planktonic foraminiferal assemblages from the Middle Eocene indurated limestones and marls collected in the External Dinarides extracted with acetic acid of different concentrations (50%, 60%, 70% and 80%) and different reaction (exposure) times. The deposits originated within the Dinaric foreland basin, have been assigned to the so-called Transitional beds and Flysch, and are characterized by different ratio of carbonate content and degree of lithification. The aim of this paper is to compare the efficiency of the laboratory procedures for obtaining isolated specimens and to evaluate the impact of preparation procedure on the quality of tests (complete test vs. secondary dissolution effects). For each acetic concentration we assessed:(1)the effectiveness of the treatment in terms of the time required for successful extraction of planktonic foraminifera, and (2)the degree of dissolution by analyses of dissolution proxies, including the weight percentage of sieved residues after disaggregation and preservation features of the tests. Our results indicate that accurate taxonomic analysis of carbonate rocks requires the use of 60% acetic acid for a shorter reaction time, and hydrogen peroxide methods for marls.

Freshwater network ecosystems consist of interconnected lotic and lentic environments within the same catchment area. Using Plitvice Lakes as an example, we studied the changes in environmental conditions and microbial communities (bacteria and fungi) that occur with downstream flow. Water samples from tributaries, interlake streams, connections of the cascading lakes, and the Korana River, the main outflow of the system, were characterized using amplicon sequencing of bacterial 16S rRNA and fungal ITS2 genes. Our results show that different environmental conditions and bacterial and fungal communities prevail among the three stream types within the freshwater network ecosystem during multiple sampling seasons. Microbial community differences were also confirmed along the longitudinal gradient between the most distant sampling sites. The higher impact of "mass effect" was evident during spring and winter, while "species sorting" and "environmental selection" was more pronounced during summer. Prokaryotic community assembly was majorly influenced by deterministic processes, while fungal community assembly was highly dominated by stochastic processes, more precisely by the undominated fraction, which is not dominated by any process. Despite the differences between stream types, the microbial community of Plitvice Lakes is shown to be very stable by the core microbiome that makes up the majority of stream communities. Our results suggest microbial community succession along the river-lake continuum of microbial communities in small freshwater network ecosystems with developed tufa barriers. IMPORTANCE Plitvice Lakes represent a rare freshwater ecosystem consisting of a complex network of lakes and waterfalls connecting them, as well as rivers and streams supplying water to the lake basin. The unique geomorphological, hydrological, biogeochemical, and biological phenomenon of Plitvice Lakes lies in the biodynamic process of forming tufa barriers. In addition to microbial communities, abiotic water factors also have a major influence on the formation of tufa. Therefore, it is important to understand how changes in environmental conditions and microbial community assembly affect the functioning of the ecosystem of a freshwater network with developed tufa barriers.

Gravel pits are considered potentially hazardous in terms of groundwater quality protection as they represent an open part of the aquifer system, increasing the aquifer’s vulnerability to contamination from the surface. The aim of this research was to determine the biogeochemical processes in gravel pits that have a positive effect on the groundwater quality in the alluvial aquifer in NW Croatia. The aquifer is situated below developed agricultural land, with high groundwater nitrate concentrations having been recorded over the last decades. The differences between two gravel pits and the surrounding groundwater were studied using in situ, hydrochemical, and isotopic parameters (δ15N-NO3 and δ18O-NO3), together with existing microbial data. The analyses of nitrogen species indicated that nitrate attenuation processes take place in gravel pits. Bacterial denitrification and nitrate uptake by algae were responsible for significant decreases in nitrate concentration. These processes were more effective in the inactive gravel pit, which has a longer water residence time and during warm periods, when microbial biomass, abundance, and activity were high. The seasonally variable microbial activity also affected trace metals, removing them from groundwater, possibly through the biosorption of metal ions. The presented research shows that the observed biogeochemical processes are associated with seasonal changes that affect the types and number of microbial communities and the chemical composition of water, resulting in gravel pits being groundwater remediation points.

Physical and chemical weathering, together with biological and biochemical processes, form soil from bedrock and strongly influence the chemical composition of natural waters. Erosive processes, primarily through the agents of running water and wind, remove the products of weathering from catchments. The aim was to determine the chemical weathering of minerals because of changes in land-use and natural forestation in two small neighboring catchments of the rivers Argilla and Bazuja. Agricultural land-use practice is very intense in the Argilla catchment, while the Bazuja catchment’s arable land is mostly abandoned, with progressive forestation. Chemical weathering in soils and sediments was evaluated with the aid of bulk chemistry analysis focused on major elements, trace elements, and zirconium. Weathering indices, mass balance, and strain were calculated. The abandonment of arable land and intense forestation in the Bazuja catchment caused increased chemical weathering with the loss of base cations (Ca and Mg) and enrichment of conservative elements (Zr and Ti) in surface horizons. EIC and MTF values are positive (enrichment) in areas with agricultural activities, while forested areas show negative values (loss). A comparison of the oldest and youngest parts of the overbank sediment profiles in the swallow hole zone and stream sediments shows that chemical and mechanical weathering in the Bazuja catchment was similar to present weathering in the Argilla catchment, while agriculture was active in the Bazuja catchment. The integrated knowledge gained in small catchment studies can be broadly applicable to larger systems.

Emerging organic contaminants (EOCs) have become of increasing interest due to concerns about their impact on humans and the wider environment. Karst aquifers are globally widespread, providing critical water supplies and sustaining rivers and ecosystems, and are particularly susceptible to pollution. However, EOC distributions in karst remain quite poorly understood. This study looks at the occurrence of EOCs in the Croatian karst, which is an example of the "classical" karst, a highly developed type of karst that occurs throughout the Dinaric region of Europe. Samples were collected from 17 karst springs and one karst lake used for water supply in Croatia during two sampling campaigns. From a screen of 740 compounds, a total of 65 compounds were detected. EOC compounds from the pharmaceutical (n = 26) and agrochemical groups (n = 26) were the most frequently detected, while industrials and artificial sweeteners had the highest concentrations (range 8-440 ng/L). The number of detected compounds and the frequency of detection demonstrate the vulnerability of karst to EOC pollution. Concentrations of 5 compounds (acesulfame, sucralose, perfluorobutane sulfonate, emamectin B1b, and triphenyl phosphate) exceeded EU standards and occurred at concentrations that are likely to be harmful to ecosystems. Overall, most detections were at low concentrations (50 % <1 ng/L). This may be due to high dilution within the exceptionally large springs of the Classical karst, or due to relatively few pollution sources within the catchments. Nevertheless, EOC fluxes are considerable (10 to 106 ng/s) due to the high discharge of the springs. Temporal differences were observed, but without a clear pattern, reflecting the highly variable nature of karst springs that occurs over both seasonal and short-term timescales. This research is one of a handful of regional EOC investigations in karst groundwater, and the first regional study in the Dinaric karst. It demonstrates the need for more frequent and extensive sampling of EOCs in karst to protect human health and the environment.

Topusko is the second warmest natural thermal water spring area in Croatia, located at the southwest edge of the Pannonian Basin System. Due to favourable geothermal properties, these waters have been used for heating and health and recreational tourism since the 1980s. Thermal springs with temperatures up to 50 °C are the final part of an intermediate-scale hydrothermal system. However, systematic research on the Topusko spring area has not been conducted to lay the foundation for sustainable resource utilisation. Multidisciplinary research including the hydrogeochemical characterisation of naturally emerging thermal water, an electrical resistivity tomography (ERT) investigation conducted to reconstruct the subsurface geology, and hydrogeological parametrisation of the geothermal aquifer was carried out to refine the existing local conceptual model. The results show Ca-HCO3 facies of Topusko thermal waters, which get heated in a Mesozoic carbonate aquifer. The water equilibrium temperature in the geothermal aquifer is estimated to be 78 °C based on the SiO2-quartz geothermometer. The fault damage zone, which enables the upwelling of thermal water, was identified by ERT investigations. The transmissivity values of the aquifer derived from the results of step-drawdown tests range from 1.8 × 10−2 to 2.3 × 10−2 m2/s. Further multidisciplinary research is necessary to improve the existing conceptual model of the Topusko hydrothermal system.

Hot spring biofilms are stable, highly complex microbial structures. They form at dynamic redox and light gradients and are composed of microorganisms adapted to the extreme temperatures and fluctuating geochemical conditions of geothermal environments. In Croatia, a large number of poorly investigated geothermal springs hosts biofilm communities. Here, we investigated the microbial community composition of biofilms collected over several seasons at 12 geothermal springs and wells. We found biofilm microbial communities to be temporally stable and highly dominated by Cyanobacteria in all but one high-temperature sampling site (Bizovac well). Of the physiochemical parameters recorded, temperature was found to have the strongest influence on biofilm microbial community composition. Besides Cyanobacteria , the biofilms were mainly inhabited by Chloroflexi , Gammaproteobacteria and Bacteroides . In a series of incubations with Cyanobacteria -dominated biofilms from Tuhelj spring, and Chloroflexi - and Proteobacteria -dominated biofilms from Bizovac well, we stimulated either chemoorganotrophic or chemolithotrophic community members, to determine the fraction of microorganisms in biofilms dependent on organic carbon ( in situ predominantly produced via photosynthesis) versus energy derived from geochemical redox gradients (here simulated by addition of thiosulfate). We found a surprisingly similar level of activity in response to all substrates in these two distinct biofilm communities and observed microbial community composition and hot spring geochemistry to be poor predictors of microbial activity in the studies systems.