Recent Research

As of the early 2000’s, there remained several unanswered, uninvestigated, or otherwise imperfectly understood questions in relation to the salmon’s role in riparian nutrient dynamics (Niaman, et al., 2002). What are the pathways for MDN to enter an aquatic system? What are the exact consequences of MDN on community composition? How exactly are MDN redistributed within a system? These are a few of the questions that have been driving recent research in the field.  

                A 2007 study set out to assess the role of Sockeye salmon in Southwestern Alaska, with the goal of determining if these fish act as net importers or exporters of nutrients (Moore, et al., 2007). By comparing estimations for incoming nutrients via salmon numbers to outgoing nutrients due hydrologic discharge of suspended sediments, it was unexpectedly revealed that salmon may in fact remove more nutrients from a system than they import.  Salmon use their tails to disturb benthic environments to create spawning nests in which to lay their eggs. By measuring quantities of suspended sediments during spawning season, it was shown in many cases using statistical analysis that bio-turbation, along with excretion from the salmon, removes more P and N than the fish bring in via their bodies. A subsequent study that compared 10 different streams in Alaska with varying levels of returning salmon across several years showed that streams with low salmon run densities showed a stable algal and insect profile (Moore, et al., 2008). Streams that had large salmon densities were shown to disturb the plankton community negatively, with a decrease of 75-80% in algal biomass. Furthermore, the algal communities in these rivers were not positively affected by the addition of salmon carcasses.

A male salmon, probably already spawned out. 

                These results are in direct contrast to the idea that salmon are net importers of nutrients to streams, prompting much research reevaluating the role salmon play in their freshwater ecosystems. An assessment of 24 streams in Alaska showed that periphyton abundance was negatively correlated with salmon densities (Verspoor, et al., 2010).  Bio-turbation during spawning season salmon disturbed benthic environments, allowing for hydrologic discharge of nutrients. This was congruous with the previous research (Moore, et al., 2008). Yet another study identified a correlation between the scale of the export of nutrients during spawning season with geomorphology, showing that rocks that are greater than 110 mm in size serve as refuges for algal communities (Holtgrieve and Schindler, 2010).  This would lead one to hypothesize that streams with ‘large’ rock substratum would respond positively to salmon nutrient import. However, streams supporting salmon runs had a general geomorphology of less than 12% of ‘large’ rock size, allowing for net loss of biomass to the algal community.  The bio-turbation effect was so important to the nutrient cycle in streams that it can actually changes streams from net autotrophic (primary production outweighs respiration) to heterotrophic (respiration outweighs primary production), regardless of salmon density index ( Holtgrieve and Schindler, 2011).  This density independence is in contrast to some studies, which propose that only low density salmon runs act as net exporters of MDN (Moore, et al., 2011).  Primary production may also be outpaced by invertebrate consumption rates and fungal growth, further reducing algal biomass accrual post spawning season (Cram, et al., 2011).  These results, supported by other studies showing similar phenomena, indicate that the ecological role that salmon play in annual nutrient pulsing may have been misunderstood until recently (Tiegs, et al., 2011. Moore, et al., 2011). 

This is a picture of a female Salmon clearing a patch of gravel to deposit her eggs. These sorts of actions (bioturbation) have been shown to possibly export more nutrients from the system than the salmon bring in through their carcass.