Estuarine Seascape Research Articles

Flood effects on estuarine fish are mediated by seascape composition and context

Estuaries are crucial feeding, nursery and resting sites for fish but can also be subject to the impacts of severe flooding. The environmental features of estuaries can mediate how they respond to these impacts. For example, the size, configuration, and context of estuarine habitats across seascapes affects the value of patches for fish, and so fish assemblages at sites with a greater habitat extent or closer to the mouth of an estuary may rebound more quickly from flooding. We investigated how a once in 100-year flood event affected fish assemblages at approximately 600 sites across 13 estuaries and six estuarine habitats (bare sediments, log snags, mangrove forests, rocky structures, saltmarsh and seagrass meadows) in southeast Queensland, Australia, and determined whether flood impacts were mediated by the position of sites within the broader estuarine seascape. Sites were surveyed annually in 2020/2021 (pre-flood) and 2022 (6 months post-flood) using underwater videography. Flooding modified the structure of the fish community and reduced the abundance of fish targeted by local fisheries in all six habitats. Crucially, flood effects on fish were greater at sites near more expansive urbanisation in some ecosystems, but lower at sites nearer to the estuary mouth. Maximising the extent of natural habitats across estuaries can mediate the effects of floods and should be priorities for restoration and management plans seeking to maintain biodiversity and fisheries productivity in the face of increasing climate-related disturbances.

Multiple Fish Species Supplement Predation in Estuaries Despite the Dominance of a Single Consumer

Predation is important in maintaining the community structure, functioning and ecological resilience of estuarine seascapes. Understanding how predator community structure, seascape context and habitat condition combine to influence predation is vital in managing estuarine ecosystems. We measured relationships between predator species richness, predator abundance and individual species abundances as well as seascape context and habitat condition, on relative predation probability in mangrove forests, seagrass meadows and unvegetated sediment across 11 estuaries in Queensland, Australia. Predation was quantified using videoed assays of tethered invertebrates (i.e. ghost nippers, Trypaea australiensis) and fish assemblages were surveyed using remote underwater video systems. Yellowfin bream (Acanthopagrus australis) dominated predation in all three habitats; however, predation was not correlated with yellowfin bream abundance. Instead, predation increased fourfold in mangroves and threefold in unvegetated sediment when predatory species richness was highest (> 3 species), and increased threefold in seagrass when predator abundance was highest (> 10 individuals). Predation in mangroves increased fourfold in forests with a lower pneumatophore density (< 50/m2). In seagrass, predation increased threefold at sites that had a greater extent (> 2000 m2) of seagrass, with longer shoot lengths (> 30 cm) and at sites that were closer to (< 2000 m) the estuary mouth. Predation on unvegetated sediment increased threefold when more extensive salt marshes (> 15000 m2) were nearby. These findings demonstrate the importance of predator richness and abundance in supplementing predation in estuaries, despite the dominance of a single species, and highlight how seascape context and habitat condition can have strong effects on predation in estuaries.

Trophic Structure of Temperate Australian Oyster Reefs Within the Estuarine Seascape: a Stable Isotope Analysis

Trophic Structure of Temperate Australian Oyster Reefs Within the Estuarine Seascape: a Stable Isotope Analysis

Seines and baited remote underwater video document consistent spatiotemporal patterns in nekton communities of subtropical coastal habitats

Monitoring nekton using multiple types of gear over a range of estuarine habitats is necessary for assessing changes in ecosystems, the effects of natural and anthropogenic disturbances, and management related to habitat restoration and protection. To assess the utility of 2 types of gear (seines and baited remote underwater videos [BRUVs]) used in many monitoring programs, nekton community metrics were compared across 3 adjacent, hydrologically distinct coastal basins in Florida Bay (Florida, USA) across seasons (wet vs. dry) and 3 water-years (2016-2019). In these basins, projects are underway to restore freshwater inflow. Spatiotemporal patterns in catch per unit effort and species richness were consistent among types of gear, and nekton assemblages differed significantly among basins. Although similar in size and spatial area, the 3 study basins were notably different in habitat (hydrology, water depth, substrate, submerged aquatic vegetation), and 1 basin exhibited distinct basin-specific trends in nekton community structure and lower nekton abundance and species richness across types of gear. Temporal differences in nekton assemblages reflected seasonal shifts and were strongly affected by the passage of Hurricane Irma in 2017. Although seines and BRUVs revealed similar patterns, seine surveys captured more overall species diversity and prey species, while BRUVs could be used in a greater variety of habitats and documented more predators. These results demonstrate the effectiveness of complementary types of gear in tracking variation in estuarine communities and highlight the importance of a sampling design that successfully monitors species throughout the food web and estuarine seascape.

European flounder foraging movements in an estuarine nursery seascape inferred from otolith microchemistry and stable isotopes

Despite the importance of estuarine nurseries in the regulation of many fish stocks, temporal and spatial movements and habitat use patterns of juvenile fish remain poorly understood. Overall, combining several movement metrics allowed us to characterize dispersal patterns of juvenile flounder, Platichthys flesus, along an estuarine seascape. Specifically, we investigated otolith microchemistry signatures (Sr:Ca and Ba:Ca ratios) and stable isotope ratios (δ13C and δ15N) in muscles of these juveniles, during three consecutive years to assess inter-annual fluctuations in their home range and isotopic niches. The morphological condition and lipid content of individuals were lower in years of high as compared to low dispersal along the estuarine gradient. We discuss these results in relation to the ecosystem productivity and intra- and inter-specific competition level, which in turn affects movements and foraging behaviors of juvenile flounders.

Trophodynamics of Nekton Assemblages and Relationships with Estuarine Habitat Structure Across a Subtropical Estuary

Understanding the spatiotemporal dynamics of trophic structures, or trophodynamics, is important for assessing the overall condition and function of estuarine ecosystems. We examined spatial and seasonal trophodynamics of nekton assemblages across the seascape of a subtropical estuary (Charlotte Harbor, FL, USA) using historical nekton data from an ongoing monitoring program. We also examined relationships between trophodynamics and the spatial distribution of vegetated habitats, such as salt marshes, mangroves, and seagrass beds, as well as their absence along hardened shorelines (seawalls or riprap) using seascape metrics (area and edge density of habitats within several search radii of sample locations). We found that nekton assemblages in shallow waters (from the shoreline to a depth of ≤ 1.5 m) transitioned from mainly planktivores in the lower Peace and Myakka Rivers to mainly benthivores (those feeding on benthic infauna, epifauna, or seagrass) in the middle and lower portions of Charlotte Harbor. Overall nekton assemblages throughout the estuary appeared to be largely sustained through benthic pathways in areas with abundant seagrass and nearby mangrove habitat, particularly during the cool-dry winter when planktivore densities were seasonally low. The dramatic seasonal decline in planktivore densities we found in the lower rivers may be related to low freshwater inflow and increased predation resulting from seasonal recruitment pulses of several omnivorous species. Trophodynamics linked with seascape metrics of estuarine habitat structure described here may be used as a spatially explicit baseline of the estuarine seascape to monitor ecosystem health as well as parameterize spatial ecosystem models used in fisheries management in Charlotte Harbor and similar subtropical estuaries.

Remnant oyster reefs as fish habitat within the estuarine seascape

Interest in oyster reef conservation and restoration is growing globally, but particularly in Australia, it is unclear the extent to which oyster reefs complement (versus replicate) habitat provisioning by other structured habitats in the seascape. Remote underwater video surveys of two east Australian estuaries revealed that at high tide, oyster reefs not only supported distinct fish communities to bare sediments but also to adjacent seagrass beds and mangrove forests. Fish observations in oyster reefs were close to double that of mangroves and seagrass, with species richness, abundance, feeding and wandering behaviours similar. Several species of blenny and goby were unique to oyster reefs and oyster-containing mangroves, whilst recreationally fished species such as bream and mullet were more abundant on oyster reefs than in other habitats. Resolving the association between oyster reefs and fish species within the broader seascape will assist in developing restoration and management strategies that maximise fisheries benefit.

Dredging fundamentally reshapes the ecological significance of 3D terrain features for fish in estuarine seascapes

ContextLandscape modification alters the condition of ecosystems and the structure of terrain, with widespread impacts on biodiversity and ecosystem functioning. Seafloor dredging impacts a diversity of flora and fauna in many coastal landscapes, and these processes also transform three-dimensional terrain features. The potential ecological significance of these terrain changes in urban seascapes has, however, not been investigated.ObjectivesWe examined the effects of terrain variation on fish assemblages in 29 estuaries in eastern Australia, and tested whether dredging changes how fish associate with terrain features.MethodsWe surveyed fish assemblages with baited remote underwater video stations and quantified terrain variation with nine complementary metrics (e.g. depth, aspect, curvature, slope, roughness), extracted from bathymetry maps created with multi-beam sonar.ResultsFish diversity and abundance were strongly linked to seafloor terrain in both natural and dredged estuaries, and were highest in shallow waters and near features with high curvature. Dredging, however, significantly altered the terrain of dredged estuaries and transformed the significance of terrain features for fish assemblages. Abundance and diversity switched from being correlated with lower roughness and steeper slopes in natural estuaries to being linked to features with higher roughness and gentler slopes in dredged estuaries.ConclusionsContrasting fish-terrain relationships highlight previously unrecognised ecological impacts of dredging, but indicate that plasticity in terrain use might be characteristic of assemblages in urban landscapes. Incorporating terrain features into spatial conservation planning might help to improve management outcomes, but we suggest that different approaches would be needed in natural and modified landscapes.

Seascape heterogeneity modifies estuarine fish assemblages in mangrove forests

AbstractMangroves are a dominant structural habitat within tropical and subtropical estuaries that provide a number of ecosystem services, including habitat for a range of crustaceans and fish. However, mangroves are one of the most threatened estuarine habitats globally, having been severely reduced in extent, and replaced by urban structures. Here, we test for the effects of both natural (e.g. seagrass, rock and mangroves habitat extent, and connectivity) and human (e.g. extent of urban area) landscape variables on the number and type of fish inhabiting mangroves forests. We used remote underwater video stations to quantify fish assemblages within mangroves at 150 sites in 30 estuaries across Queensland, Australia. Fish community structure was best explained by the extent of mangroves and seagrasses within an estuary, the distance to the estuary mouth, and the size of the estuary and catchment. Moderate catchment size and proximity to the estuary mouth increased species richness and abundance of harvestable fish at individual mangrove sites. In order to maintain mangrove fish assemblages and the functions they provide, management initiatives should focus on maintaining natural estuarine seascapes that are located closer to the mouth of estuaries, in particular, focusing on estuaries that have lower levels of catchment urbanization.

Low redundancy and complementarity shape ecosystem functioning in a low-diversity ecosystem.

Ecosystem functioning is positively linked to biodiversity on land and in the sea. In high-diversity systems (e.g. coral reefs), species coexist by sharing resources and providing similar functions at different temporal or spatial scales. How species combine to deliver the ecological function they provide is pivotal for maintaining the structure, functioning and resilience of some ecosystems, but the significance of this is rarely examined in low-diversity systems such as estuaries. We tested whether an ecological function is shaped by biodiversity in a low-diversity ecosystem by measuring the consumption of carrion by estuarine scavengers. Carrion (e.g. decaying animal flesh) is opportunistically fed on by a large number of species across numerous ecosystems. Estuaries were chosen as the model system because carrion consumption is a pivotal ecological function in coastal seascapes, and estuaries are thought to support diverse scavenger assemblages, which are modified by changes in water quality and the urbanization of estuarine shorelines. We used baited underwater video arrays to record scavengers and measure the rate at which carrion was consumed by fish in 39 estuaries across 1,000km of coastline in eastern Australia. Carrion consumption was positively correlated with the abundance of only one species, yellowfin bream Acanthopagrus australis, which consumed 58% of all deployed carrion. The consumption of carrion by yellowfin bream was greatest in urban estuaries with moderately hardened shorelines (20%-60%) and relatively large subtidal rock bars (>0.1km2 ). Our findings demonstrate that an ecological function can be maintained across estuarine seascapes despite both limited redundancy (i.e. dominated by one species) and complementarity (i.e. there is no spatial context where the function is delivered significantly when yellowfin bream are not present) in the functional traits of animal assemblages. The continued functioning of estuaries, and other low-diversity ecosystems, might therefore not be tightly linked to biodiversity, and we suggest that the preservation of functionally dominant species that maintain functions in these systems could help to improve conservation outcomes for coastal seascapes.

Assessing trophic flexibility of a predator assemblage across a large estuarine seascape using blood plasma stable isotope analysis

The northcentral Gulf of Mexico (nGoM) encompasses a range of environments that support a speciose predatory assemblage. Large predatory species are often viewed as trophically-analogous, sharing similar isotopic niches. To investigate the regional and seasonal isotopic variability of a predator assemblage across the coastal waters of the nGoM (Louisiana, Mississippi, and Alabama), we analyzed the stable isotopes of carbon, nitrogen, and sulfur using a fast turnover tissue, blood plasma, for seven dominant predator species. Carbon, nitrogen, and sulfur isotope values varied regionally, with a significant interaction between region and season for nitrogen and sulfur. Species' isotopic niches varied regionally, as well as seasonally, leading to varying levels of isotopic niche overlap among species. Blacktip sharks Carcharhinus limbatus and Atlantic sharpnose sharks Rhizoprionodon terraenovae in particular demonstrated flexibility in their isotopic relationships, whereas blacknose sharks C. acronotus maintained a more consistent isotopic niche regardless of region or season. No biologically significant overlap was observed for blacknose sharks, bull sharks C. leucas, or spinner sharks C. brevipinna, suggesting that these species occupy distinct isotopic space. Overlap calculations using two isotopes (carbon and nitrogen) produced markedly higher overlap versus three isotopes (carbon, nitrogen, and sulfur), which demonstrates that predatory roles may be oversimplified when using only carbon and nitrogen. These findings highlight the value of examining seasonal variation in trophic roles using fast turnover tissues and provide the first triple-isotope characterization of a common predatory assemblage in the northcentral Gulf of Mexico.

Contrasting effects of mangroves and armoured shorelines on fish assemblages in tropical estuarine seascapes

AbstractCoastal seascapes are composed of a diversity of habitats that are linked in space and time by the movement of organisms. The context and configuration of coastal ecosystems shapes many important properties of animal assemblages, but potential seascape effects of natural and artificial habitats on nearby habitats are typically considered in isolation. We test whether, and how, the seascape context of natural and urban habitats modified fish assemblages across estuaries. Fish were sampled with underwater videos in five habitat types (mangroves, rock bars, log snags, unvegetated sediments, armoured shorelines) in 17 estuaries in eastern Australia. Different habitats supported distinct fish assemblages, but the spatial context of mangroves and armoured shorelines had pervasive ecological effects that extended across entire estuaries. In most estuarine habitats, fish diversity and abundance was greatest when they were in close proximity of mangroves, and decreased due to the proximity of armoured shorelines. Many cities are centred on estuaries, and urban expansion is often associated with the fragmentation of mangrove forests. Our findings emphasize that these transformations of urban estuarine landscapes are likely to propagate to broader ecological impacts detectable in multiple habitats beyond mangrove forests.

Movement ecology of a mobile predatory fish reveals limited habitat linkages within a temperate estuarine seascape

Large predatory fishes, capable of traveling great distances, can facilitate energy flow linkages among spatially separated habitat patches via extended foraging behaviors over expansive areas. Here, we tested this concept by tracking the movement of a large mobile estuarine fish, red drum (Sciaenops ocellatus). Specifically, we addressed the following two questions: (i) What are the spatial and temporal patterns of red drum movement (rates of dispersal) and activity space? (ii) Does red drum movement facilitate linkages among estuarine marsh complexes? Dispersal from the release location was greatest during the first 2 weeks at liberty before declining to less than 0.5 km·week–1 for the remainder of the study. Activity space initially increased rapidly before reaching an asymptote at 2.5 km2 2 weeks postrelease. Connectivity indices calculated among marsh complexes corroborated these observations, suggesting high residency and limited seascape-scale linkages via red drum movement behaviors. These data highlight potential within-estuary spatial structure for mobile fishes and could inform subsequent efforts to track energy flows in coastal food webs, predict the footprint of local habitat restoration benefits, and enhance the design of survey regimes to quantify overall population demography.

Maximizing the benefits of oyster reef restoration for finfish and their fisheries

AbstractGlobal declines in oyster reefs have resulted in reduced habitat heterogeneity, extent and quality for some coastal finfish, potentially reducing fish populations and catches. It is well established that habitat restoration results in higher finfish biomass and diversity where oyster reefs replace bare substrata. Therefore, restoring oyster reefs with a view to also improving fish stocks is often a key goal of oyster restoration. However, the principles of habitat quality, ecological connectivity and broader ecosystem management are poorly integrated within oyster reef restoration ecology, but such principles may be instructive in enhancing the benefits of projects on fish populations throughout estuarine seascapes. This manuscript presents a framework for projects seeking to restore both oyster reef habitat and finfish communities. Structurally and biologically complex oyster reefs, comprising both oysters and other invertebrates, are required to provide shelter, food and nursery services to fish. By carefully considering site selection at seascape scales (km to 10s of km), restoration can enhance the network of habitat available to fish and potentially increase the overall carrying capacity of the estuary. Managers of estuaries that now include restored oyster reefs should implement fisheries management plans and consider the effects of management actions broadly throughout catchments; failing to do so may jeopardize gains in fish yields. Management decisions must be adaptable, responding to key criteria in thorough monitoring programs. Integrating these ecological and coastal management concepts into oyster reef restoration will enhance outcomes for fishes and increase stakeholder engagement and cost‐effectiveness.

Dynamic habitat use of an estuarine nursery seascape: Ontogenetic shifts in habitat suitability of the European flounder (Platichthys flesus)

Estuaries have long been recognized as nursery habitats for various marine fish species. As seascapes consisting of a mosaic of habitat patches, estuarine nurseries incorporate ecological processes such as ontogenetic shifts. To improve our understanding of the habitat requirement changes of nursery species within the estuarine seascape, habitat suitability models were built using boosted regression trees for three size classes of European flounder (Platichthys flesus) juveniles. The models combined several dynamic and static predictors, from physical and chemical water parameters to biological and physical habitat variables. The relationships between juveniles and predictor variables varied with size, potentially indicating the dynamics of habitat use for the juvenile flounder inside the estuarine nursery. Younger post-settled juveniles tended to be spatially constrained to a shallow upstream area and, with size, there were ontogenetic shifts to the middle estuary. This demonstrates the importance of considering different life stages when developing essential fish habitat maps, which reflect fine scale temporal and spatial distributions. Given the high level of anthropogenic pressures to which estuaries are subjected, the consideration of fish ontogenetic habitat requirements within the estuarine nursery is needed to identify high-value areas, and is valuable for developing effective spatial and temporal management plans. These are essential to fully protect the most sensitive life stages, such as early post-settlement stages occupying transient habitats. Additionally, and as the juvenile-habitat associations can rapidly change, fine-scale connectivity in the estuarine nursery becomes even more relevant in determining recruitment. Connectivity should therefore be central in conservation planning, in order to sustain estuarine and marine adult populations, and ultimately maintaining the fisheries.

Investigating the Functional Role of an Artificial Reef Within an Estuarine Seascape: a Case Study of Yellowfin Bream (Acanthopagrus australis)

Estuaries contain mosaic habitats which support fish across different life stages. Artificial reefs represent a form of habitat enhancement which can provide additional structure for fishes and improve fishing opportunities, but the role of artificial reefs within the broader estuarine seascape has not been extensively studied. We used a VEMCO Positioning System (VPS) to monitor the fine-scale movements of yellowfin bream (Acanthopagrus australis, referred to as Bream), an estuarine predator and important recreational species. Fish were implanted with acoustic tags with accelerometer sensors (to measure relative fish activity), and their movements monitored on an artificial reef and adjacent habitats. Elevated activity patterns during crepuscular periods indicated that foraging was likely occurring over a large seagrass bed adjacent to the artificial reef system. Alternatively, lower activity was observed when fish were on the artificial reef, which may reflect the role of this habitat as a refuge, or that alternative foraging strategies were being employed. All fish exhibited a high degree of fidelity to the artificial reef on which they were tagged, and there was minimal movement among other reef groups within the array. There was extensive overlap in space use contours for smaller fish on the seagrass edge, but no overlap for larger fish that also tended to forage further afield. These findings have implications for the way in which artificial reefs support fish production, especially the importance of connectivity with other key habitats within the estuarine seascape.

Seagrass meadows shape fish assemblages across estuarine seascapes

Estuarine seascapes are a mosaic of habitat types that are connected, to varying degrees, by the movement of organisms. In these seascapes, the attributes of the habitats themselves, as well as the spatial context in which they are embedded, can shape faunal assemblages. Here, we explicitly test how habitat and connectivity interact to shape fish assemblages across estuaries. Our model system comprised 6 distinct estuarine habitats (log snags, mangroves, rocky outcrops, seagrass, sand/mud and urban structures), which represent the full diversity of estuarine habitats and seascape contexts in our study systems. Fish were sampled with cameras at 318 sites in 13 subtropical estuaries in eastern Australia. Habitat type was the principal predictor of fish assemblage composition, an effect that was shaped by spatial connections to other habitats, particularly seagrass beds. Seagrass structured fish assemblages, not only by providing a complex habitat that contained more species and individuals than all other habitats, but also by having a 'footprint' beyond the edge of meadows. Fish were more diverse and abundant in non-seagrass sites that were closer to seagrass meadows. Two other seascape attributes, proximity of sites to the sea and to mangroves, also influenced the composition of fish assemblages, albeit less consistently than distance to seagrass. Conservation and fisheries management programs that seek to enhance or restore fish populations in subtropical estuaries should prioritise seagrass conservation initiatives, which encompass critical ecological linkages with other habitats in estuarine seascapes.

Movement ecology of a mobile predatory fish reveals limited habitat linkages within a temperate estuarine seascape

Movement ecology of a mobile predatory fish reveals limited habitat linkages within a temperate estuarine seascape

Macro-Restoration of Tidal Wetlands: A Whole Estuary Approach

A large-scale wetland restoration case-study is discussed in response to fish losses in an open cycle, cooling water system at a generating facility located on Delaware Bay, USA. Stable isotope analyses of vegetation, resident and marine transient finfishes in marshes and open waters of the estuary are described, along with biochemical condition of individuals as it relates to habitat quality, and secondary production. Population dynamics of spot ( Leiostomus xanthurus ), a “target species” impacted by the generating facility was used to compare fish losses at the intake with new production of this species in the restored marshes. A “whole estuary” (or seascape) approach to restoration was adopted, one that integrates the concepts of donor control, linkages between tidal salt marshes, the marsh-estuary-coastal continuum and the recruitment success of marine transients. We emphasize that individual wetlands do not function in isolation; rather they are spatially explicit and functionally connected habitat mosaics incorporating ecological processes driven by organism behavior. Linkages among habitats that affect the growth and survival of earlier life stages therefore tend to be underplayed in restoration planning; but few species are confined to a single habitat; e.g., tidal salt marshes. In contrast, the findings of our seascape focused study demonstrated consistent and predictable animal density or productivity ‘hotspots’ in relation to spatial position within the seascape. Both ontogenetic habitat shifts, the use of transitory and temporary habitats, and the concept of the estuarine seascape are discussed in the context of restoring not just habitats, but also estuarine-coastal “connectivity”.

Habitat- and bay-scale connectivity of sympatric fishes in an estuarine nursery

Acoustic telemetry was used to examine habitat- and bay-scale connectivity for co-occurring juvenile fishes, southern flounder (Paralichthys lethostigma) and red drum (Sciaenops ocellatus), at two spatial scales in a model estuarine seascape. An acoustic positioning system was deployed to examine habitat-scale (ca. 1 m–1 km) movement, while a larger gridded array was deployed to examine bay-scale movement (ca. 1–20 km). Both species exhibited greater use of edge habitat and seagrass beds at the habitat scale; however, rates of movement within habitats varied between species. Southern flounder movement (mean = 4.0 m min−1) increased with decreasing habitat complexity (seagrass to bare sand) and increasing temperature, while red drum rate of movement (mean = 8.4 m min−1) was not significantly affected by environmental factors at the habitat scale, indicating the use of different foraging strategies (i.e. ambush vs. active). Bay-scale distribution was influenced by physicochemical conditions and seascape composition, with both species found most frequently in areas with high seagrass coverage and relative close proximity to tidal creeks and connective channels. Response to environmental variables often differed between species and the probability of bay-scale movement (>1 km) for southern flounder was greatest on days with narrow tidal ranges (<0.4 m) and higher temperatures (>17 °C), while the probability of bay-scale movement for red drum increased in response to decreasing salinity and lower temperatures (<16 °C). Species-specific variation in movement patterns within and across habitat types observed here at both the habitat and bay scale suggest sympatric species employ different strategies to partition resources within estuarine nursery areas and highlight the importance of multi-species assessments for improving our understanding of habitat value and ecosystem function.