Aquatic plants regulate downstream nutrients

Posted on March 26, 2022

Just as the processes of respiration and nutrient uptake by living cells affect the chemistry of the human body, the activities of all life-forms in a body of water influence its metabolism and carbon dynamics.  In freshwater streams, aquatic plants and algae are important agents of metabolism because they act as the stream’s internal source of primary production.  However, until recently no studies on annual metabolism had been conducted in streams with abundant waterplants.  To fill this knowledge gap, biologists measured primary production and ecosystem respiration in a temperate stream in Jutland, Denmark over an entire year.  The stream was in an agricultural catchment with an area of 72 km2..  The researchers compared two segments of stream that were 1.5 km apart:  the upstream stretch had little aquatic vegetation (less than 5% cover), but the downstream stretch had a cover of more than 60% in summer.  The main water plants were lesser water-parsnip (Berula erecta), water-starwort (Callitriche sp.) and water-crowfoot (Ranunculus aquatilis).  Rates of stream metabolism were measured by recording the upstream-downstream difference in dissolved oxygen over a distance of 158 m in the high vegetation reach and 274 m in the low vegetation reach.  Daily gross primary production was calculated as the sum of dissolved oxygen produced in daylight hours plus the oxygen consumed over the same period, which was estimated from nighttime respiration.  Data analysis showed that annual primary production was twice as high in the vegetated reach as in the low vegetation reach.  In the two reaches production was at a similar low in winter, but summer production was about 35 and 17 times higher in the high and low vegetation reaches respectively.  In the high vegetation reach production stayed unexpectedly high through autumn, but in the low vegetation reach it dropped markedly in mid-summer, perhaps because seasonally heavy shading by grasses and shrubs in summer inhibited benthic algal production.  Over the year ecosystem respiration was 1.3 times greater in the high vegetation reach.  These findings demonstrate that waterplants can have a strong influence on stream metabolism.  However, it was significant that respiration outweighed production throughout the year, even in the well-vegetated reach.  This was due to the retention and decomposition of newly-produced organic matter in stands of vegetation, as well as the breakdown of abundant organic material washed in from the catchment.  It seems that beds of waterplants can delay the downstream transport of nutrients from summer to winter, and thus help to prevent low-oxygen conditions developing further downstream.

Reference:   Alnoee, A.B. et al.  2021.  Macrophytes enhance reach‑scale metabolism on a daily, seasonal

and annual basis in agricultural lowland streams.  Aquatic Sciences 83:11