Stream nitrate enrichment and increased light yet no algal response following forest harvest and experimental manipulation of headwater riparian zones

Abstract

Disturbances to forested watersheds often result in increases of nutrients and light to nearby streams. Such changes are generally expected to produce a shift to a more autotrophic aquatic ecosystem, with measurable increases in algae, and associated implications for food webs and fisheries. Although this paradigm is widely established, results from our 10-year study (2007–2016) in 12 headwater streams and four sites downstream in the Trask River Watershed (Oregon, USA), did not concur. In 2012, one watershed was thinned, three were clearcut harvested with variable buffers and three with uniform riparian buffers. After harvest, light to the stream surface significantly increased at the three watersheds with variable buffers while dissolved inorganic nitrogen (DIN) significantly increased in all of the clearcut harvested streams. Despite the increase in DIN and light, algal standing stocks and chlorophyll a concentrations did not significantly increase. The common assumption of increased autotrophic responses in stream food webs following increases of nitrogen and light was not supported here. We postulate the co-limitation of nutrients, driven by low phosphorus concentrations, which unlike DIN did not increase post-harvest, and the characteristics of the algal community, which were dominated by low light adapted diatoms rather than green algae, contributed to our findings of no responses for standing stocks of epilithic algae or concentrations of chlorophyll a. The inclusion of multiple statistical analyses provided more certainty around our findings. This study documents responses to current forest practices and provides cautionary information for management and restoration activities aiming to increase fish abundance and standing stocks by opening riparian canopies and adding nutrients.

Experimental riparian forest gaps and increased sediment loads modify stream metabolic patterns and biofilm composition

“…Similarly, biofilm biomass in the natural streams was two times higher in the light sites than in the dark sites. This difference in biomass between shaded and open channels was higher than reported from a similar study in Finland (Jyväsjärvi et al, 2022) and Oregon, USA (Johnson et al, 2023). This is likely due to the observed increase in filamentous green algae in our clear-cut sites and unshaded channels, in line with, for example, Lowe et al (1986).…”

“…This is likely due to the observed increase in filamentous green algae in our clear-cut sites and unshaded channels, in line with, for example, Lowe et al (1986). Filamentous algae are not always captured by artificial tiles, such as those used by Jyväsjärvi et al (2022), and growth of green algae was not promoted in Johnson et al (2023). Ultimately, large increases in filamentous algae can negatively affect macroinvertebrate diversity (Tonkin et al, 2014).…”

“…In contrast, streams in forests managed by rotational forestry differ from old-growth forests in that they typically experience dramatic changes during and after clear-cutting, with increases in incident light and stream temperatures (Chellaiah & Kuglerov a, 2021;Roon et al, 2021;Warren et al, 2013), possible pulses of nutrients (Futter et al, 2010;Kreutzweiser et al, 2008), and increases in sediment loading and suspended particles (Palviainen et al, 2014;Rachels et al, 2020). Streams draining clear-cuts therefore can experience high primary production but low species diversity and low nutritional quality of biofilms-which often make up the base of the food web (e.g., Lowe et al, 1986;Noel et al, 1986), unless limited by nutrients (Johnson et al, 2023) or degraded by high sediment export (Murphy & Hall, 1981). On the contrary, during later successional stages, between 30 and 100 years, streams shift toward highly shaded, low-nutrient conditions, leading to dramatic decreases in aquatic productivity (Burrows et al, 2021;Murphy & Hall, 1981).…”