Dark matters: Contrasting responses of stream biofilm to browning and loss of riparian shading

Abstract

Concentrations of terrestrial‐derived dissolved organic carbon (DOC) in freshwater ecosystems have increased consistently, causing freshwater browning. The mechanisms behind browning are complex, but in forestry‐intensive regions browning is accelerated by land drainage. Forestry actions in streamside riparian forests alter canopy shading, which together with browning is expected to exert a complex and largely unpredictable control over key ecosystem functions. We conducted a stream mesocosm experiment with three levels of browning (ambient vs. moderate vs. high, with 2.7 and 5.5‐fold increase, respectively, in absorbance) crossed with two levels of riparian shading (70% light reduction vs. open canopy) to explore the individual and combined effects of browning and loss of shading on the quantity (algal biomass) and nutritional quality (polyunsaturated fatty acid and sterol content) of the periphytic biofilm. We also conducted a field survey of differently colored (4.7 to 26.2 mg DOC L−1) streams to provide a ‘reality check’ for our experimental findings. Browning reduced greatly the algal biomass, suppressed the availability of essential polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA), and sterols, but increased the availability of terrestrial‐derived long‐chain saturated fatty acids (LSAFA). In contrast, loss of shading increased primary productivity, which resulted in elevated sterol and EPA contents of the biofilm. The field survey largely repeated the same pattern: biofilm nutritional quality decreased significantly with increasing DOC, as indicated particularly by a decrease of the ω‐3:ω‐6 ratio and increase in LSAFA content. Algal biomass, in contrast, was mainly controlled by dissolved inorganic nitrogen (DIN) concentration, while DOC concentration was of minor importance. The ongoing browning process is inducing a dramatic reduction in the nutritional quality of the stream biofilm. Such degradation of the major high‐quality food source available for stream consumers may reduce the trophic transfer efficiency in stream ecosystems, potentially extending across the stream‐forest ecotone.

The emerging role of drought as a regulator of dissolved organic carbon in boreal landscapes

“…For headwater environments (e.g., 1st and 2nd order streams), low flow responses can also include widespread hypoxic conditions and accumulation of reduced inorganic solutes 6 , which likely have first-order influences on aquatic organisms and community structure 40 . By comparison, for streams, rivers, and lakes embedded further down in the aquatic network, summer DOC declines are likely to have more direct ecological and biogeochemical impacts, given the role of DOC as an energy source to aquatic heterotrophs 41 , as a contributor to watercolor that restricts aquatic photosynthesis 10 and mediator of the nutritional quality of algal resources to consumers 42 . Indeed, average summer DOC concentrations for the largest stream (C16) varied by nearly three-fold over the 17 years (5.2−14.6 mg L −1 ), a range comparable to that which is thought to shape aquatic metabolic patterns across broad regional gradients in the northern hemisphere 43 .…”

Drainage‐induced browning causes both loss and change of benthic biodiversity in headwater streams

“…Our data showed a gradual reduction in invertebrate richness with increasing DOC, and an abrupt shift in benthic macroinvertebrate community composition when DOC concentrations reached 12-13 mg L À1 . Importantly, algal scrapers were the only feeding group with a strong negative response to DOC, likely related to an indirect effect of DOC on invertebrates via their key food source, benthic biofilm, which is of lower nutritional quality at higher DOC levels (Jyväsjärvi et al 2022). The importance of the indirect linkage between invertebrate consumers and browning via food resources is further supported by the observation that other feeding groups (whose food resources should be less affected by browning) showed a weaker (gathering-collectors; predators) or no response (shredders) to increasing DOC.…”

“…The importance of the indirect linkage between invertebrate consumers and browning via food resources is further supported by the observation that other feeding groups (whose food resources should be less affected by browning) showed a weaker (gathering-collectors; predators) or no response (shredders) to increasing DOC. Jyväsjärvi et al (2022) combined a mesocosm experiment and field survey to show a drastic reduction in algal (particularly diatom) biomass and bioavailability of polyunsaturated fatty acids (PUFA) with water browning. At the same time, the availability of terrestrial-derived long-chain saturated fatty acids poor in PUFAs (Taipale et al 2016) increased.…”

“…We first classified streams into three subgroups based on DOC-concentration: low (< 10 mg L À1 ; n = 19), moderate (10-15 mg L À1 ; n = 19) and high-DOC (> 15 mg L À1 ; n = 25), roughly corresponding to 20%, 75%, and 98% of water color observations in ca. 4500 Finnish streams (Jyväsjärvi et al 2022). Environmental gradients were then visualized with principal component analysis (PCA), separately for habitat and water chemistry variables.…”

“…It thus appears that browning may exert at least some, and sometimes quite strong, control over benthic consumers but this effect is mainly indirect, reflecting the negative effect of browning on benthic algal productivity. It is also possible that algal quality, not quantity, is decisive; the nutritional quality of benthic algae decreases already at the early stages of browning, even if algal quantity shows little response (Jyväsjärvi et al 2022). Browning is associated with several concurrently changing factors and the individual effects of, for example, increased DOC and nutrients (e.g., Bergström and Karlsson 2019) cannot be distinguished without experimentation.…”