Diel patterns in stream nitrate concentration produced by in-stream processes

Diel variability in stream NO3- concentration represents the sum of all processes affecting NO3- concentration along the flow path. Being able to partition diel NO3- signals into portions related to different biochemical processes would allow calculation of daily rates of such processes that would be useful for water quality predictions. In this study, we aimed to identify distinct diel patterns in high-frequency NO3- monitoring data and investigated the origin of these patterns. Monitoring was performed at three locations in a 5.1 km long stream reach draining a 430 km2 catchment. Monitoring resulted in 355 complete daily recordings on which we performed a k-means cluster analysis. We compared travel time estimates to time lags between monitoring sites to differentiate between in-stream and transport control on diel NO3- patterns. We found that travel time failed to explain the observed lags and concluded that in-stream processes prevailed in the creation of diel variability. Results from the cluster analysis showed that at least 70 % of all diel patterns reflected shapes typically associated with photoautotrophic NO3- assimilation. The remaining patterns suggested that other processes (e.g., nitrification, denitrification, and heterotrophic assimilation) contributed to the formation of diel NO3- patterns. Seasonal trends in diel patterns suggest that the relative importance of the contributing processes varied throughout the year. These findings highlight the potential in high-frequency water quality monitoring data for a better understanding of the seasonality in biochemical processes.

Home range characteristics and diel patterns in space use of mutton snapper, Lutjanus analis, in St. Thomas, US Virgin Islands

Background The movement ecology of mutton snapper Lutjanus analis is poorly understood despite their ecological and economic importance in the Caribbean. Passive acoustic telemetry was used to determine home ranges of six adult L. analis, including diel patterns, in Brewers Bay, St. Thomas, US Virgin Islands. Understanding long-term space use, including site fidelity and habitat usage, is necessary to implement effective and appropriate management actions for a species with extensive space and resource needs. Results Individual L. analis were tracked over an average period of 316 days (range 125–509 days) and showed high site fidelity to relatively small home ranges (mean ± SD: 0.103 ± 0.028 km2, range 0.019–0.190 km2) and core use areas with low overlap among individuals. Most home ranges had a habitat composition dominated by seagrass and to a lesser degree, coral reef and/or pavement. Nighttime activity spaces were distinct from but contained within daytime areas. Conclusions Mutton snapper showed strong site fidelity to home ranges in Brewers Bay. Two individuals that were absent from the array for more than a few hours were detected at separate arrays at spawning aggregation sites. This study expands upon knowledge of mutton snapper home range characteristics, highlights the importance of maintaining adjacent high-quality habitat types in any spatial management plan, and encourages the adoption of other types of management strategies, particularly for transient-aggregating species.

Diel patterns in nitrate concentration suggest importance of microbial pathways for in-stream processing

Diel cycles in stream nitrate concentration represent the sum of all processes affecting nitrate concentration along the flow path. Being able to partition diel nitrate signals into portions related to different biochemical processes would allow to calculate daily rates of such processes that are urgently needed for water quality predictions. In this study we analyzed diel nitrate patterns at three locations in a 5.1 km long stream reach draining a 430 km2, mainly forested but anthropogenically influenced catchment during one growing season. We tested if the observed diel variability in nitrate concentration resulted from upstream sources and subsequent downstream transport or emerged simultaneously along the stream. We determined time lags between monitoring sites by cross-correlation. We found that time lags were closer to zero than travel time estimation assuming plug-flow suggested and concluded that ubiquitous in-stream processes prevailed in the creation of diel variability. To further analyze the diel nitrate signals we used k-means clustering to identify patterns in the diel portion of nitrate concentrations and interpreted the resulting clusters with regard to potential drivers and the calculated nitrate balance of sub-reaches. We found that 70 % of all diel patterns were attributed to clusters negatively related to the diel course of insolation with highest nitrate amplitudes on warm and sunny days and low water levels. We argue that temporal shifts towards the remaining clusters are rather due to shifts in microbial nitrate processing than in photosynthesis-driven plant uptake. These results suggest that the magnitude of microbial nitrate processing may be large compared to plant uptake.

Home Range Characteristics and Diel Patterns in Space Use of Mutton Snapper, Lutjanus Analis, in St. Thomas, Us Virgin Islands

Background: The movement ecology of mutton snapper Lutjanus analis is poorly understood despite their ecological and economic importance in the Caribbean. Passive acoustic telemetry was used to determine home ranges of six adult L. analis, including diel patterns, in Brewers Bay, St. Thomas, US Virgin Islands. Understanding long-term space use, including site fidelity and habitat usage, is necessary to implement effective and appropriate management actions for a species with extensive space and resource needs.Results: Individual L. analis were tracked over an average period of 316 days (range 125 - 509 days) and showed high site fidelity to relatively small home ranges (mean ± SD: 0.103 ± 0.028 km2, range 0.019 - 0.190 km2) and core use areas with low overlap among individuals. Most home ranges had a habitat composition dominated by seagrass and to a lesser degree, coral reef and/or pavement. Nighttime activity spaces were distinct from but contained within daytime areas.Conclusions: Mutton snapper showed strong site fidelity to home ranges in Brewers Bay. Two individuals that were absent from the array for more than a few hours were detected at separate arrays at spawning aggregation sites. This study expands upon knowledge of mutton snapper home range characteristics, highlights the importance of maintaining adjacent high-quality habitat types in any spatial management plan, and encourages the adoption of other types of management strategies, particularly for transient-aggregating species.