This food web is for the upper reaches of the San Marcos River. Fewer Fountain Darters downstream and no Roundnose Minnows. Less Biofilms and
macrophytes too because of substrate differences. (BOTTOM OF THE RIVER)
Note water bird dotted line to parasitic worm (a trematode). “Dotted” line means transfer (through digestive tract). The parasite moves into a snail (step not
shown in web; for simplicity) and leaves snail and moves into fishes.
Note Suckermouth Catfish: no predators (at least on the bigger fish), hence the concern of the non-native fish and no top-down check on population numbers.
Background: Among numerous Edward/Trinity spring complexes sampled throughout the Edwards Plateau, spring-associated fishes dominant the community
from headwater springs to confluences with rivers (turbid, not thermally constant like spring waters) (see attached Craig et al. 2016). River fishes are dominant
in the rivers.
Based on Behen (2013, TxState these; publication forthcoming), we found spring-associated fishes dominated the upper San Marcos to Ed Capes dam, but were not the most abundant downstream
of Ed Capes dam to confluence of Blanco River. River fishes were. Hence we concluded that the lower San Marcos River (Ed Capes dam to confluence) is not meeting expectations of a spring fish
community (expectations come from Craig et al. 2016).
In Kollaus et al. 2015 (towards the end of the publication), we provide three plausible reasons (theories) on why lower San Marcos River was not meeting expectations to support a spring fish
community. In order of importance:
1). Cummings Dam: Raises water level unnaturally in the lower reach of San Marcos River. Deep water is not necessarily a limiting factor for Fountain Darters (they are found in Spring Lake up to
20 ft in water depth), but the water is a little murkier (Groeger et al. see attached), partially related to discharge of water from the waste water treatment plant (higher nutrients than the receiving
water) and light doesn’t penetrate the water as much because of tree canopy cover, which might or might not be “natural”. Reduced light restricts plant growth, therefore small amounts of plants
downstream from Ed Capes. Remediation: remove Cummings Dam, lower water levels, allow more plants to grow, though plant growth still might be restricted by canopy cover. However, lower
water levels mean greater water velocities, which means the stream channel might become more mobile. More mobility means trees could be washed out, etc., creating more open canopy (which
I believe is the more natural condition).
2) Ed Capes Dam: Per Kollaus et al, Ed Capes and all other dams on the San Marcos River trap sediments upstream from the dam. Another problem downstream of the dam is the lack of silt
substrates for macrophyte growth (in addition to low light levels). I view the lower river as “sediment starved”. Allowing sediments to move downstream, rather than trapped behind Ed Capes
dam, would provide heterogeneous mix of substrates, rather than dominated by the impenetrable Taylor Marl strata layer that underlies the area. More sediments means more veg growth and
more darters.
3) Waste water treatment plant: As described above, adds more nutrients, which are taken up by phytoplankton and algae and reduces light.
Steps for remediation of the lower reach:
Option 1: Take down Cummings and remove Ed Capes. Ignore waste water treatment plant because water residency time will be reduced without Cummings Dam.
Option 2: Take down only Cummings Dam (not an option at this time). Would likely have the greatest remediation effect. However, the reach might be sediment starved because of Ed Capes, so
could yield “no effect”.
Option 3. Take down only Ed Capes: Allows more sediment downstream. However, water level with Cummings Dam could negate the benefit of Ed Capes Removal.
Removing Ed Capes has the potential to allow more spring-associated fishes downstream by providing the sediment for macrophyte growth and allow more fountain darters in the area. Or it will
have no effect unless Cummings is removed. Or, it will have no effect because Kollaus et al (I’m part of this group, so therefore…”my”) perspectives are wrong.
Removing Ed Capes now and later Cummings (it will be removed at some point, though it might be 50 years down the road) are plausible steps to recovery. Putting Ed Capes back in as a true dam
and not a flow through like Rio Vista could negate the future benefits of Cummings removal.
Rebuilding Capes dam: We’ll not have an effect on the current San Marcos River Fountain Darter population. Things are really good in the San Marcos River (upstream of the dam) for the Fountain
Darters. Rebuilding the dam maintains status quo.
I will not argue against all of the “cons” of dam removal. I’ve heard several that lack any validity. However, I will address one: Fountain Darters and Texas Wildrice numbers will decrease in the
area upstream of Ed Capes dam after removing the dam. This is true. Without the dam, water level will drop (stranding some wildrice; less slack water for Fountain Darters) and current velocities
will increase (reducing habitat for Fountain Darters and likely wildrice). However, losses will be compensated with rehabilitation of the lower reach, with the potential of producing far greater
numbers of Fountain Darters and Wildrice (if all of our theories are correct). Therefore, worth the risk, in my opinion.