Evaluating the Food Web and Ecosystem Function of a Large, Shallow, Eutrophic Lake Dominated by Non Native Fishes

Keywords

Restoration; Ecopath; Food Web Model; Ecosystem Function; Ecosystem Network Analysis; Utah Lake; Cyprinus Carpio

Abstract

Utah Lake, Utah, USA, has undergone an anthropogenic ecosystem shift from clearwater to a turbid state in less than 150 years. There is now strong citizen support for its restoration, including its native fishes. However, no effort has been made to understand its food web or ecosystem function, essential for scientifically based restoration. Consequently, a representative food web model was urgently needed. We present the first-ever food web-based, ecosystem function model of Utah Lake using EcoPath. Biomasses, diets, and production/biomass ratios of eighty-one taxonomic groups were modeled. Results showed an impaired Utah Lake ecosystem dominated by only a handful of taxa, including invasive carp, other invasive fishes, and pollution-tolerant chironomids. Lake energy sources were co-dominated by water column primary production and detritus, mostly as detrital snow with negligible benthic photosynthetic production. Utah Lake has low robustness (e.g., resistance), is below optimal trophic functioning, and is trapped in an ‘immature’ early succession stage primarily because of chronic wave action that disturbs unconsolidated sediments, thus preventing system maturation, maintaining the lake in a ‘bloom’ susceptible eutrophic state that underutilizes available nutrients. Carp reduction had positive mixed-level trophic impacts on several groups, indicating the need for an improved reduction program. Increased monitoring and research are essential, including ecosystem network analyses needed to predict future changes to Utah Lake’s ecosystem, including directing restorative actions.

Abstract

Utah Lake, Utah, USA, has undergone an anthropogenic ecosystem shift from clearwater to a turbid state in less than 150 years. There is now strong citizen support for its restoration, including its native fishes. However, no effort has been made to understand its food web or ecosystem function, essential for scientifically based restoration. Consequently, a representative food web model was urgently needed. We present the first-ever food web-based, ecosystem function model of Utah Lake using EcoPath. Biomasses, diets, and production/biomass ratios of eighty-one taxonomic groups were modeled. Results showed an impaired Utah Lake ecosystem dominated by only a handful of taxa, including invasive carp, other invasive fishes, and pollution-tolerant chironomids. Lake energy sources were co-dominated by water column primary production and detritus, mostly as detrital snow with negligible benthic photosynthetic production. Utah Lake has low robustness (e.g., resistance), is below optimal trophic functioning, and is trapped in an ‘immature’ early succession stage primarily because of chronic wave action that disturbs unconsolidated sediments, thus preventing system maturation, maintaining the lake in a ‘bloom’ susceptible eutrophic state that underutilizes available nutrients. Carp reduction had positive mixed-level trophic impacts on several groups, indicating the need for an improved reduction program. Increased monitoring and research are essential, including ecosystem network analyses needed to predict future changes to Utah Lake’s ecosystem, including directing restorative actions.

Keywords

Restoration; Ecopath; Food Web Model; Ecosystem Function; Ecosystem Network Analysis; Utah Lake; Cyprinus Carpio

Citation

Richards DC, Miller TG (2025) Evaluating the Food Web and Ecosystem Function of a Large, Shallow, Eutrophic Lake Dominated by Non-Native Fishes. Ann Environ Sci Ecol 5(1): 22.

INTRODUCTION

Utah Lake (40°12’59.96”N, 111°48’28.20”W), Utah, USA, a remnant of Late Pleistocene Lake Bonneville, is a large (≈380 km2), shallow (mean ≈3 m depth), turbid, eutrophic, slightly saline, and highly regulated lake managed as a reservoir. Its ecosystem has been severely degraded since first settlement by Americans of European descent over 150 years ago [1]. Primary production in Utah Lake has been almost completely dominated by water column phytoplankton (algae) for more than 50 years. In a healthy state, the lake should have relatively equal contributions from water column and benthic primary production given its shallow nature. Algal blooms, including potentially harmful cyanobacteria blooms (HABs), occasionally occur during some summer months. Submerged and emergent aquatic macrophytes (plants) were abundant throughout much of its once extensive littoral zone but are now near absent. Mollusk (mussels, clams, and snails) diversity and abundance peak in the Utah Lake-Jordan River drainage and the surrounding areas in the molluskan depauperate western USA [2,3]. Unfortunately, Utah Lake’s keystone native mollusk community has been near extirpated. The lake’s once diverse benthic invertebrate assemblage is now mostly dominated by only two or three pollution tolerant chironomid (midge) and oligochaete (segmented worm) taxa [1]. Utah Lake once supported 13 species of native fish, ten of which have been extirpated or are extinct, including the keystone species Bonneville Cutthroat Trout (Oncorhynchus virginalis utah). The lake currently only supports three native species including Threatened June Sucker (Chasmistes liorus), Utah Sucker (Catastomus ardens), and Utah Chub (Gila artraria) that account for less than 2% relative abundance whereas, twelve non-native and highly invasive species now account for 98% relative abundance, including over dominance by Common Carp (Cyprinus carpio) (S2). By most standards the lake has lost its ecological integrity and is in poor health. Utah Lake’s resilience to future perturbation and its resistance to improvement (restoration) appears to be compromised [1]. Given its degraded state, there is much concern by citizens and managers as to the future of Utah Lake and what can be done to improve its condition (i.e., health, integrity), including the reduction of algal blooms and restoring native fish populations. However, the focus of concern has been almost exclusively on external nutrient input reduction and to a lesser extent invasive carp control. There has been little to no effort expended to examine or understand the importance of direct and indirect interactions within the lake’s food web or how top-down, trophic cascades directly and indirectly effect and respond to current conditions. Managers are just now considering how biomanipulation may help restore its ecosystem functions. Restoring Utah Lake’s ecosystem function and native fisheries cannot proceed without this understanding. Hence, a representative food web- based, ecosystem function model was urgently needed. Presented here is the first trophic- level, mass- balanced food web model of Utah Lake. This model along with subsequent iterations and improvements will allow researchers and managers to understand and better manage this important ecosystem. A food web reference point is now established for Utah Lake for comparisons with future scenarios. Describing the food web structure and trophic interactions among the lakes’ biological groups will vastly improve our current knowledge on the role of key ecosystem processes and development of decision support tools necessary for effective management [4-6].

METHODS

Food web models are widely used to investigate complex interactions at the ecosystem level by providing simplified representations based on trophic structure. One of the most popular approaches to determine aquatic food web dynamics is the temporal dynamic modeling approach used in EcoPath with EcoSim (EwE) [7,8]. EwE models have been shown to provide valuable forecasts [9-11]. The EwE modelling approach is based on modelling energy flow and provides several well documented flow based indicators [8-13]. EwE has been applied to hundreds of ecosystems around the world [14,15], with over 500 unique models documented in EcoBase, so far (see website at http://sirs.agrocampus-ouest.fr/ EcoBase/). EwE has been used for large lake food web modelling including extensively throughout the Great Lakes, USA [16-23]. Derivations of EwE are detailed in Christensen and Pauly and Walters et al.; software and documentation are available at http://www.EcoPath.org/. EwE version 6.6.7 was used for our Utah Lake food web model. 

Two master equations must be satisfied to correctly parameterize the EcoPath model. The first equation describes the production of each functional group as a set of n linear equations for n groups:

where (Pi /Bi ) is the production to biomass ratio for group i (which is equal to the coefficient of total mortality Z under steady-state conditions [24], EEi is the ecotrophic efficiency (the proportion of production used in the system), Bi and Bj are the biomasses of the prey and predators respectively, (Qj /Bj ) is the consumption to biomass ratio, DCji is the fraction of prey i in predator j’s diet, Yi is catch rate for the fishery for group i, Ei is the net migration rate, and Bai is the biomass accumulation for group i.

The EcoPath model assumes conservation of mass over a year. Energy balance within each group is ensured with the second master equation:

Consumption = production + respiration + unassimilated energy where production can be described as:

Production = predation mortality + catches + net migration + biomass accumulation + other mortality

where Pi is the production of prey group i, Qj is the consumption of predator j, DCji is the diet composition contribution of i to j’s diet, Fi is the instantaneous rate of fishing mortality, NMi is the net migration rate of prey group i, BAi is the biomass accumulation rate for i, M0i is the other mortality rate for i (non-predation, non-fishery), and Bi is the biomass of i.

Data Used

Eighty-one taxa groups were used in the model (Table 1a, 1b). Phytoplankton, zooplankton, and benthic invertebrate biomass and diets were derived from Richards and Miller [25-28], Richards [1-32], Wasatch Front Water Quality Council, and Utah Division of Water Quality databases. Fish biomass and diets were derived from Walsworth, Landom, and Gaeta [33], Walsworth and Landom [34], Walsworth, Wallace, and Landom [35], University of Utah unpublished diet estimates, and Utah Division of Wildlife Resources survey data. Fish taxa biomass other than carp  biomass estimates were derived from reported carp biomass estimates [34], relative abundances, and length/weight relations presented in S1 and S2. Fish diet data were cross referenced using FishBase (https:// www.fishbase.se/home.htm). Fish taxa were separated into several ontogenetic groups based on University of Utah unpublished diet study size classes (Table 1b). Zooplankton and benthic invertebrate ontogenetic groups were not used in this model but can be in future models. Basic estimates of the food web inputs are in Table 2. Production to biomass ratio (P/B) estimates were generated from literature values. Most of the phytoplankton, zooplankton, and benthic invertebrate P/B values (Table 2) were estimated on the high end of reported values because Utah Lake is highly eutrophic and productive. For example, phytoplankton P/B was consistent with Mutsert et al. [36], where P/B = 182 in nutrient rich waters of the Gulf of Mexico that had high primary production [37]. Benthic invertebrate P/B was modeled based on Richards unpublished data and freshwater benthic invertebrate authorities Benke et al., and Hauer and Benke who documented that chironomid P/B > 100 is common [38]. Several diet estimates had to be adjusted incrementally to balance the model because EcoPath is a mass balance model, and all diet proportions must add to 1.00. Utah Lake mean annual temperature of 16.7 OC [1], was used to help generate and verify several benthic invertebrate production values using Brey [39], associated computational spreadsheets, empirical models, and the Brey [40] model.

Model Parameters

The model was based on yearly averages using data from 1995 to 2021, although seasonal averages can be modeled in future iterations when adequate data are compiled, and the need arises. The currency unit used was wet weight (tons/km2). The model was developed as a mostly quasi-closed system, i.e., little or no migration but can be modeled as an open system (e.g., transfer of mass and energy in and out of the system) in future iterations. Utah Lake ecosystem was not separated into subregions for this model but can be in future scenarios.

Fleets: There is only one semi-commercial fishing fleet in Utah Lake, the carp removal project. Estimates of tons km-2 year-1 of carp removal vary and were tentatively modeled at 3 t km-2 year-1 based on Walsworth, Wallace, and Landom [35]. Unfortunately, the carp removal project was put on hold starting in 2024. Recreational sports fisheries are popular on Utah Lake. However, no estimates of ‘take’ (i.e., creel census data) have been collected by fisheries managers therefore, we estimated harvest as 5% of harvestable larger size class game fish biomass, excluding two non game fish, Fathead Minnow and June Sucker.

Exports, Immigration and Emigration rates: Export estimates were made for two Chironomidae taxa, Chironominae and Tanypodinae as adults leaving the system. We assumed that about half of the adult population biomass of these two taxa did not return to lake based on several assumptions, 1) males were half the adult population biomass and did not return as did females to deposit eggs, 2) wind events forced a portion of the population away from the lake, 3) an unknown proportion of the adult population was lost to predation (e.g., spiders, birds, dragonflies, etc.), and 4) artificial urban lights attracted adults away from the lake and that portion of the population was unable to return to the lake. Subsequently net migration was estimated at 1.70 t km2 y-1 for Chironominae and 0.52 t km2 y-1 for Tanypodinae. We estimated immigration for detritus at 1% that mostly occurs during high water spring runoff. As far as we know, no empirical data has ever been collected for immigration of other biota into Utah Lake and was assumed to be negligible given our knowledge of the ecosystem. Certainly, some plankton, drifting invertebrates, and fishes leave (emigrate) the system downstream via the Jordan River. Emigration in Utah Lake occurs ‘naturally’ when water levels are higher than the

dam typically during spring runoff but is becoming more uncommon. Emigration occurs during lower water levels thorough out much of the remainder of the year via a pump station for water users downstream. There were no empirical data available for estimation of these exports for any of the groups used in this model. Therefore, we estimated emigration rates of 10% for phytoplankton, zooplankton, detrital bacteria, and detrital snow. We assumed emigration of other taxa was negligible.

RESULTS

Diets and metrics

S2 contains diet estimates for all taxa groups. In addition to these taxa diets, two frog species, Boreal Chorus Frog (Pseudacris maculata) (native) and the American Bullfrog (Lithobates catesbeianus) (invasive) occur in Utah Lake but there was no available biomass data consequently, they were not used in the model. However, one American Bullfrog was found in a Northern Pike stomach sample. Table 2 provides EcoPath’s basic estimates of Utah Lake food web including estimated biomass (ton km-2), production/biomass (year-1) and production/consumption (year 1). Consumption/biomasses (year-1) and ecotrophic efficiencies (EE) were estimated by the EcoPath model. Figures 1, 2 and 3 show trophic level flows within the food web based on biomass and production, respectively. Table 3 provides three key indices, flow to detritus (tons km2 year-1), net efficiency, and omnivory index from the EcoPath model. Our model estimated that unconsumed cyanophytes could contribute more than 100 tons km-2 yr-1 to detritus whereas bacillariophytes (diatoms) contributed about 2 tons km-2 yr-1 (Figure 4).

Figure 1: Trophic level flow diagram of Utah Lake food web based on biomass (tons km-2).

Figure 2: Trophic level flow diagram of Utah Lake food web based on production (tons km-2 year -1).

Table 3: Flow to detritus, net efficiency, and omnivory index