Invasive macrophytes induce context‐specific effects on oxygen, pH, and temperature in a hydropeaking reservoir

Abstract

Dense macrophyte beds are known to produce extreme diurnal oxygen and temperature conditions in shallow lakes. However their influences in managed hydropeaking reservoirs has received limited attention. We measured dissolved oxygen, pH and water temperature in the Lake Kar apiro hydroreservoir, northern New Zealand, across a gradient of proportional water-column height occupied by the invasive macrophytes Egeria densa and Ceratophyllum demersum, which dominated in the upperriverine (variable water inflow) and lower-lacustrine (variable water level) sections, respectively. Hypoxia and anoxia events that occurred inside invasive macrophyte beds during their summer peak biomass accumulation period were more pronounced for C. demersum than for E. densa, and within the bottom 20% of the water column. In contrast, pH and temperature changed little in relation to proportional macrophyte height. Macrophyte species differences in the production of hypoxia and anoxia events increased when site-specific hydropeaking management covariates (depth, inflows, water level) were accounted for. This association with hydropeaking likely resulted from contrasting hydrodynamics in the lower-lacustrine and upper-riverine lake sections, where oxygen can decrease with higher water levels and lower water inflow rates, respectively. During the course of our study, some macrophyte beds were treated with herbicide, enabling us to document prolonged and sustained hypoxic/anoxic conditions near the bottom following spraying. These results underscore the adverse effects of invasive macrophytes on water physicochemical attributes that sustain aquatic biota, and highlight the context-dependent nature of these effects moderated by reservoir management for hydropeaking and macrophyte control.

Hydrology-mediated invasive macrophyte impacts on freshwater mussels in a hydropeaking reservoir

“…Reservoir management can exacerbate or mitigate the adverse environmental conditions produced by invasive submerged macrophytes near the lake-bed. For example, Moore et al (2020) reported higher reservoir residence time led to reduced water mixing and promoted prolonged anoxic and hypoxic conditions within submerged macrophyte beds in sections of a northern New Zealand hydropeaking reservoir (also the location of the present study in both riverine and lacustrine sections). Accordingly, hydrology (i.e., riverine or lacustrine systems) may partly account for context-specific nature of mussel responses to invasive submerged macrophyte impacts at small spatial scales.…”

“…This waterbody, the most downstream in a series of eight reservoirs, has a surface area of 5.4 km 2 , and mean and maximum depths of 11 m and 30.5 m, respectively (Lowe and Green 1987). The reservoir is considered eutrophic (Livingston 1986) with a residence time dependent on inflow: for example, minimum, mean and maximum water inflows of 208, 262 and 320 m 3 s -1 equate to residence times of 3.3, 2.6 and 2.2 days, respectively, given the assumptions of full water column mixing and a lake water volume of 60 x 10 6 m 3 ; Gibbs et al 2015, Moore et al 2020. Karāpiro has an upper-riverine section (hereafter riverine), where discharge from the Arapuni hydropower station produces highly variable flows (as above), and a lower-lacustrine section (hereafter lacustrine) that has a diurnally variable water level related to hydropeaking (see Moore et al 2020).…”

Wind Exposure Regulates Water Oxygenation in Densely Vegetated Shallow Lakes

“…At other sites, heterotrophic activity, stimulated by temperature increase during summer and autumn, exceeded net oxygen release during the day, that resulting in hypoxia/anoxia events and buildup of CO 2 , CH 4 and NH 4 + in the water column. This observation is recurrent in dense stands formed by invasive macrophytes, where the sedimentation of organic matter generates an elevated benthic BOD during the period of senescence of plants; this implies a permanent DO deficit [ 15 , 31 , 32 ]. In dense hydrophyte stands, DO input from the atmosphere can be limited to the surficial layer of the water column, as long stems constitute a physical barrier, as floating-leaved macrophytes do [ 13 ].…”

“…However, oxygen dynamics can be altered in densely vegetated stands, such as those dominated by invasive macrophytes: here, due to a fast cycle of growth and decay, fresh organic matter continuously replenishes the organic bulk in the sediment [ 11 ]. In those conditions, elevated biological oxygen demand (BOD) and hypoxia are coupled to accumulation and stratification of nutrients, such as carbon dioxide, methane, ammonium and reactive phosphorous, also during the day [ 12 , 13 , 14 , 15 ].…”

Long-Term Effects of the Harvesting of Trapa natans on Local Water Quality and Aquatic Macrophyte Community in Lake Erhai, China

“…They play a decisive role in the structure and function of aquatic plant communities in the shallow waters of Erhai Lake, and often cause a simple and unstable community structure in eutrophic waters. In addition, the biomass of C. demersum fluctuates greatly during the year (He et al, 2015), and it declines in autumn (Dong et al, 2019) and then causes dramatic fluctuations in the community structure and composition, weakening the ecological function of the community (Sayer et al, 2010;Moore et al, 2020). Relevant studies have shown that it is reasonable to maintain a low coverage of C. demersum in eutrophic water bodies (Dai et al, 2012).…”