Regime shift in the littoral ecosystem of volcanic Lake Atitlán in Central America: combined role of stochastic event and invasive plant species

Abstract

Different functional groups of macrophytes vary in their impact on aquatic ecosystem structure and processes. The introduction of new species with different growth form, combined with a stochastic event, may have serious and irreversible consequences on lake functioning. Our goals were to document and explain physical, chemical, metabolic and biotic changes in the littoral zones of a volcanic lake before and following two coinciding events: invasion by a submersed macrophyte, Hydrilla verticillata (Hydrocharitaceae), followed by a rapid increase in the lake water level (>2.5 m). We recorded plant biomass, plant tissue C:N:P stoichiometry, macroinvertebrates, water characteristics data along transects through littoral zones, and measured gas emission in controlled mesocosms and in the lake. The native emergent species, Schoenoplectus californicus (Cyperaceae), was generally not able to survive such a rapid water level increase, and Hydrilla spread and formed dense mats further preventing Schoenoplectus regeneration. The impact of another introduced species, the free‐floating Eichhornia crassipes (Pontederiaceae), was more localised, despite its much longer presence at the lake. Although the three species had comparable standing biomass, the two invader species had lower C:N:P ratios than Schoenoplectus, resulting in faster decomposition rates and indicating potential shifts in nutrient cycling within the ecosystem. The oxygen profile of the water column was altered by the non‐native species in a significantly different manner: in Eichhornia, the saturation concentrations dropped down to 30%–50% of dissolved oxygen, while oxygen supersaturation was recorded in Hydrilla. Both Schoenoplectus and Eichhornia patches exhibited comparable carbon dioxide (CO2) fluxes, sequestering 230 and 300 mg CO2 m−2 hr−1, respectively, during the day and emitting 250 and 200 mg CO2 m−2 hr−1, respectively, during the night. Contrary to these two species, Hydrilla patches sequestered CO2 during the day (34 mg CO2 m−2 hr−1) and night (44 mg CO2 m−2 hr−1). The invasive species maintained a richer community of macroinvertebrates compared to several native species (excluding Schoenoplectus), both in taxa diversity and in numbers of individuals. When the results are considered in the regional context, an increase in nutrient supply could lead to the dominance of free‐floating plants. We discussed management options more broadly considering the negative impacts of introduced species balanced against their beneficial effects, in the context of environmental changes.

Long-Term Habitat Degradation Drives Neotropical Macrophyte Species Loss While Assisting the Spread of Invasive Plant Species

“…To date, there are few studies addressing the full extent of abiotic and biotic changes that may occur in response to the spread of invasive macrophyte species in neotropical lakes (Villamagna and Murphy, 2010;Rejmánková et al, 2018). Even less is known of the ecological conditions prior to invasion and the long-term dynamics of spread linked to other cooccurring stressors such as eutrophication (Strayer et al, 2006).…”

“…The distinctive plant structure and high biomass of most invasive macrophytes can further cause strong and varied ecosystem engineering effects (Crooks, 2002;Yarrow et al, 2009). For instance, dense beds of invasive plants offer colonization space for epiphytic algae, invertebrates, and fish (Meerhoff et al, 2003), and can greatly stimulate diversity and populations of these organisms (Kelly and Hawes, 2005;Rejmánková et al, 2018). Similarly, dense stands of invasive floating plants enhance shading, reduce temperature, and cause anoxia in the underlying water that ultimately inhibit understory species (Villamagna and Murphy, 2010).…”

“…Solms, Pistia stratiotes L., Azolla filiculoides L., and Salvinia molesta Mitchell (Scheffer et al, 2003). Their effects may include reductions in the number of fish, declines in water quality, navigation obstruction (Gopal, 1987), modification of habitat structure and displacement of native macrophyte species (Yarrow et al, 2009;Rejmánková et al, 2018). The full extent of biotic and abiotic changes that may occur in response to the spread of these species is, however, poorly studied in the neotropics (Villamagna and Murphy, 2010;Rejmánková et al, 2018).…”

“…Their effects may include reductions in the number of fish, declines in water quality, navigation obstruction (Gopal, 1987), modification of habitat structure and displacement of native macrophyte species (Yarrow et al, 2009;Rejmánková et al, 2018). The full extent of biotic and abiotic changes that may occur in response to the spread of these species is, however, poorly studied in the neotropics (Villamagna and Murphy, 2010;Rejmánková et al, 2018). Even less is known about how increasing human-induced stress factors such as eutrophication and hydrological alterations (e.g., lake level fluctuations) may interact over time with the spread of invasive macrophyte species spread to jointly influence aquatic plant biodiversity (Byers, 2002;Rejmánková et al, 2018).…”

Hydrologic alterations impact plant litter decay rate and ecosystem resilience in Mojave wetlands

“…If rhizome resprouts are physically unable to break through the persistent, dense litter layer, continual "false starts" potentially lead to exhaustion of the rhizome reserves and eventual collapse of the plant patch (Klimes et al 1993;Rejmánková et al 1999Rejmánková et al , 2018. This inhibitory effect of litter as a physical barrier is often observed for seedling establishment and survival (Amatangelo et al 2008;Loydi et al 2013).…”