Effects of a diatom ecosystem engineer (Didymosphenia geminata) on stream food webs: implications for native fishes

Stream habitat changes affecting primary consumers often indirectly impact secondary consumers such as fishes. Blooms of the benthic algae Didymosphenia geminata (Didymo) are known to affect stream macroinvertebrates, but the potential indirect trophic impacts on fish consumers are poorly understood. In streams of the Kootenai River basin, we quantified the diet, condition, and growth rate of species of trout, char, and sculpin. In 2018, macroinvertebrate taxa composition was different between a stream with Didymo and a stream without, but trout diets, energy demand, and growth rates were similar. Trout abundance was higher in the stream with Didymo, but the amount of drifting invertebrates was higher in the stream without. In 2019, we surveyed 28 streams with a gradient of coverage. Didymo abundance was correlated only with the percentage of aquatic invertebrates in trout diets and was not related to diets of char or sculpin or condition of any species. Thus, we found no evidence for a trophic link between Didymo blooms and the condition or growth of trout, char, or sculpin in mountainous headwater streams.

Defining the Limits of Spectrally Based Bathymetric Mapping on a Large River

Remote sensing has emerged as a powerful method of characterizing river systems but is subject to several important limitations. This study focused on defining the limits of spectrally based mapping in a large river. We used multibeam echosounder (MBES) surveys and hyperspectral images from a deep, clear-flowing channel to develop techniques for inferring the maximum detectable depth, d m a x , directly from an image and identifying optically deep areas that exceed d m a x . Optimal Band Ratio Analysis (OBRA) of progressively truncated subsets of the calibration data provided an estimate of d m a x by indicating when depth retrieval performance began to deteriorate due to the presence of depths greater than the sensor could detect. We then partitioned the calibration data into shallow and optically deep ( d > d m a x ) classes and fit a logistic regression model to estimate the probability of optically deep water, P r ( O D ) . Applying a P r ( O D ) threshold value allowed us to delineate optically deep areas and thus only attempt depth retrieval in relatively shallow locations. For the Kootenai River, d m a x reached as high as 9.5 m at one site, with accurate depth retrieval ( R 2 = 0.94 ) in areas with d < d m a x . As a first step toward scaling up from short reaches to long river segments, we evaluated the portability of depth-reflectance relations calibrated at one site to other sites along the river. This analysis highlighted the importance of calibration data spanning a broad range of depths. Due to the inherent limitations of passive optical depth retrieval in large rivers, a hybrid field- and remote sensing-based approach would be required to obtain complete bathymetric coverage.

Nutrient uptake during low-level fertilization of a large, seventh-order oligotrophic river

Uptake of nitrogen (total nitrogen (TN), NH4-N, and NO3-N) and phosphorus (total dissolved phosphorus (TDP) and total phosphorus (TP)) was quantified June through September 2009–2011 using whole-river fertilization in a seventh-order, P-limited river (Kootenai River, Idaho, USA), at discharges up to three orders of magnitude greater than previously studied. Mean uptake length (Sw) and uptake velocity (Vf) values were similar for dosed TDP and NH4; both had steep gradients indicating rapid uptake, while NO3-N did not. TP remained higher than reference levels. TN showed no clear pattern. Autotrophs accounted for 28% of daylight mean NO3-N uptake compared with 72% by heterotrophs. Nutrient uptake was strongly associated with chlorophyll accrual and epilithon growth rates. Mean midsummer epilithon growth and N rates roughly tripled late summer rates. TDP uptake length (Sw = 5.7 km) showed a slow increase with increasing stream order consistent with published findings. Mean TDP uptake velocity (Vf = 32 mm·min−1) was eight times greater than previously seen in smaller streams. Vf (10.9 ± 5 mm·min−1) and Sw (16.8. ± 7 km) for NO3-N increased with increasing river order and discharge.