Plant-Mediated Community Structure of Spring-Fed, Coastal Rivers
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bioRxiv preprint doi: https://doi.org/10.1101/678128; this version posted June 20, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. This article is a US Government work. It is not subject to copyright under 17 USC 105 and is also made available for use under a CC0 license. 1 2 3 4 Plant-mediated community structure of spring-fed, coastal rivers 5 6 7 Matthew V. Lauretta1, William E. Pine, III2, Carl J. Walters3, Thomas K. Frazer4 8 9 10 11 1National Oceanic Atmospheric Administration, National Marine Fisheries Service, Miami, FL, 12 United States of America 13 14 2Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL, 15 United States of America 16 17 3Fisheries Research Centre, University of British Columbia, Vancouver, British Columbia, 18 Canada 19 20 4School of Natural Resources and Environment, University of Florida, Gainesville, FL, United 21 States of America 22 23 24 *Corresponding author 25 Email: [email protected] (ML) 26 1 bioRxiv preprint doi: https://doi.org/10.1101/678128; this version posted June 20, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. This article is a US Government work. It is not subject to copyright under 17 USC 105 and is also made available for use under a CC0 license. 27 Abstract 28 Quantifying ecosystem-level processes that drive community structure and function is 29 key to the development of effective environmental restoration and management programs. To 30 assess the effects of large-scale aquatic vegetation loss on fish and invertebrate communities in 31 Florida estuaries, we quantified and compared the food webs of two adjacent spring-fed rivers 32 that flow into the Gulf of Mexico. We constructed a food web model using field-based estimates 33 of community absolute biomass and trophic interactions of a highly productive vegetated river, 34 and modeled long-term simulations of vascular plant decline coupled with seasonal production of 35 filamentous macroalgae. We then compared ecosystem model predictions to observed 36 community structure of the second river that has undergone extensive vegetative habitat loss, 37 including extirpation of several vascular plant species. Alternative models incorporating bottom- 38 up regulation (decreased primary production resulting from plant loss) versus coupled top-down 39 effects (compensatory predator search efficiency) were ranked by goodness-of-fit tests of model 40 predictions to the empirical community observations. Our best model for predicting community 41 responses to vascular plant loss incorporated coupled effects of decreased primary production 42 (bottom-up), increased prey search efficiency of large-bodied fishes at low vascular plant density 43 (top-down), and decreased prey search efficiency of small-bodied fishes with increased biomass 44 of filamentous macroalgae (bottom-up). The results of this study indicate that the loss of vascular 45 plants from the coastal river ecosystem may alter the food web structure and result in a net 46 decline in the biomass of fishes. These results are highly relevant to ongoing landscape-level 47 restoration programs intended to improve aesthetics and ecosystem function of coastal spring-fed 48 rivers by highlighting how the structure of these communities can be regulated both by resource 2 bioRxiv preprint doi: https://doi.org/10.1101/678128; this version posted June 20, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. This article is a US Government work. It is not subject to copyright under 17 USC 105 and is also made available for use under a CC0 license. 49 availability and consumption. Restoration programs will need to acknowledge and incorporate 50 both to be successful. 51 Introduction 52 Human alterations of the landscape and associated changes in the physical and chemical 53 properties of aquatic environments have resulted in the loss of submersed vascular plants from 54 shallow aquatic ecosystems around the world [1, 2, 3]. For example, in nutrient-enriched 55 systems, algae capable of rapid nutrient uptake and growth can dominate and potentially exclude 56 vascular plants with lower nutrient uptake, assimilation, and growth rates [4]. Because plants 57 play a central role in the ecology of aquatic ecosystems in a variety of ways, including 58 contributing to ecosystem production, modifying biogeochemical processes, and mediating biotic 59 interactions [1, 5, 6, 7], changes in aquatic plant communities may have profound consequences 60 on ecosystem structure. Rooted vascular plants link benthic substrates to the overlying water 61 column through the uptake of nutrients from sediment pore waters and transport of organic 62 matter, minerals, and gases to both the water and benthic environments [8, 9]. The above-ground 63 structure provided by these plants also decreases sediment erosion and resuspension by reducing 64 water velocity and turbulence at the water-sediment interface [8, 10, 11]. As a consequence, 65 water clarity and light availability are increased and a positive feedback loop for vascular plant 66 primary production is provided. 67 Plants directly support the production of higher trophic levels and have been shown to 68 have a major influence on the abundance and diversity of stream faunal communities [12]. Plants 69 mediate predator-prey interactions between fishes and invertebrates by providing refuge habitat 70 for prey populations and decreasing prey encounter rates of predators [13], allowing predator and 71 prey populations to coexist at relatively high densities compared to unstructured habitats [14, 15, 3 bioRxiv preprint doi: https://doi.org/10.1101/678128; this version posted June 20, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. This article is a US Government work. It is not subject to copyright under 17 USC 105 and is also made available for use under a CC0 license. 72 16]. The effects of vascular plant loss and replacement by algal species on the faunal 73 communities that they support are not currently well-understood [17], but this replacement is 74 observed globally and is increasingly of concern to water resource users and managers [2, 3, 18, 75 19]. 76 Due to their unique hydrogeological and biological properties, Florida’s spring-fed rivers 77 are excellent model ecosystems to study the effects of vegetative habitat loss on community 78 structure. These rivers generally have stable streamflow and water temperatures, as well as 79 extremely high rates of primary production [7, 20] that support diverse communities of plants, 80 invertebrates, and fishes [21, 22]. Spring systems are also currently of significant conservation 81 concern because of increased demands on groundwater for consumptive uses, which can lead to 82 reduced spring discharge [23, 24], salinization [23], altered nutrient dynamics [25], and changes 83 in floral and faunal communities [18, 26]. Within these spring ecosystems, documented shifts in 84 the composition and biomass of primary producers are temporally concordant with changes in 85 watershed land-use, including increased agricultural and residential development, and associated 86 changes in streamflow and water quality [25, 27]. Of particular concern is the rapid decline of 87 once dominant vascular plants, including Vallisneria Americana, Potamogeton spp., and 88 Sagittaria kurziana in several systems, and the widespread proliferation of filamentous 89 macroalgae, including Chaetomorpha spp., Gracilaria spp., and Lyngbya spp. [27, 28, 29], 90 which may have effects on higher trophic levels [17]. 91 The causal mechanisms of vascular plant loss from the rivers or the effects on faunal 92 communities within these systems are not fully understood [18]. However, resource managers 93 are tasked with developing restoration and management plans for these systems, necessitating a 94 need for science to inform restoration strategies. As an example, the loss of vascular plants, 4 bioRxiv preprint doi: https://doi.org/10.1101/678128; this version posted June 20, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. This article is a US Government work. It is not subject to copyright under 17 USC 105 and is also made available for use under a CC0 license. 95 which provide forage and refuge habitat, may alter invertebrate grazer communities, predator- 96 prey dynamics of fishes, and other important population-level interactions. Such alterations may 97 lead to undesirable shifts in fish and invertebrate communities and loss of ecosystem function, 98 which is of critical importance to managers and resource users concerned with the conservation 99 of these unique coastal ecosystems [30, 31, 32]. 100 To evaluate fish and invertebrate community responses to vegetative habitat loss, we (1) 101 conducted a large-scale field assessment of ecosystem structure and function in two adjacent 102 spring-fed rivers (Chassahowitzka and Homosassa rivers) where changes in vascular plant 103 biomass (and replacement by filamentous macroalgae) have occurred with different trajectories; 104 (2) used this information to construct a food web model using empirical estimates of plant, 105 invertebrate, and fish absolute biomass and trophic interactions from the more vegetated river 106 (Chassahowitzka River); (3) simulated seasonal