Ecosystem metabolism in tropical streams and rivers: a review and synthesis

Abstract

Ecosystem metabolism of freshwater ecosystems has been studied for several decades, with theory and synthesis largely derived from temperate streams and rivers in North America and Europe. Advances in sensor technology and modeling have opened a wider range of streams to be included to test theories beyond temperate streams. In this paper, we review and synthesize ecosystem metabolism data from tropical streams and rivers to determine to what extent the constraints of metabolism measured in temperate streams are similar in tropical streams. We compiled 202 measurements of gross primary productivity (GPP) and ecosystem respiration (ER) from 83 tropical streams spanning 22.2 S to 18.4 N. Overall, tropical streams were heterotrophic (ER > GPP), with GPP ranging from 0.01 to 11.7 g O 2 m −2 d −1 and ER ranging from −0.2 to −42.1 g O 2 m −2 d −1 , similar on average to rates reviewed from temperate streams, but with higher maximum ER in tropical streams. Gross primary productivity increased with watershed area; a result also observed in temperate streams. ER decreased with elevated phosphorus and higher annual rainfall. We constructed a structural equation model that explained greater variation of ER (74%) than GPP (26%), and reflects similar drivers, such as land-use and watershed area, as in temperate streams. We conclude that tropical stream ecosystem metabolism has similar drivers as temperate streams, and a warmer and wetter climate and human use of tropical lands will influence metabolic rates in streams.

Land Use Change Influences Ecosystem Function in Headwater Streams of the Lowland Amazon Basin

“…We found very low GPP, high ER, and negative NEP in all streams, consistent with other studies reporting widespread net heterotrophy in forested tropical streams [9]. Low light availability as a result of dense riparian canopy cover in both land use types likely plays a large role in constraining photosynthesis (Figure 1; [9,77,[92][93][94]. Our GPP values were similar to other studies in the neighboring Cerrado region (<0.1-0.818 g m −2 d −1 ; [95,96]), the Peruvian Amazon (0.07-0.189 g m −2 d −1 ; [97]), and forested headwater tropical streams more broadly, which typically have very low or undetectable rates of GPP [9,92,93,98,99].…”

“…We found very low GPP, high ER, and negative NEP in all streams, consistent with other studies reporting widespread net heterotrophy in forested tropical streams [9]. Low light availability as a result of dense riparian canopy cover in both land use types likely plays a large role in constraining photosynthesis (Figure 1; [9,77,[92][93][94].…”

“…Low light availability as a result of dense riparian canopy cover in both land use types likely plays a large role in constraining photosynthesis (Figure 1; [9,77,[92][93][94]. Our GPP values were similar to other studies in the neighboring Cerrado region (<0.1-0.818 g m −2 d −1 ; [95,96]), the Peruvian Amazon (0.07-0.189 g m −2 d −1 ; [97]), and forested headwater tropical streams more broadly, which typically have very low or undetectable rates of GPP [9,92,93,98,99]. Rates of ER ranged from 0.1-11.9 g m −2 d −1 , which was similar to studies from the Cerrado (0.12-19.58 g m −2 d −1 ; [95,96,99], Amazon (1.8-15. Although we found no land use or seasonal differences in rates of GPP, it was more frequently measurable and, thus, higher in cropland streams than forested streams (Tables S7 and S8, Figure 5), similar to studies showing greater GPP in agricultural streams [92,100,101].…”

“…Globally, nearly 7.8 million hectares of native forest are cleared every year (average for 1990-2015), most of which occurs along agricultural frontiers in the mega-diverse wet tropics [8]. Despite this, we know relatively little about the effects of land use change on streams in lowland tropical areas compared with those in temperate regions [9][10][11]. Understanding these effects is increasingly important because the lowland tropics support species-rich forests and savannas [12], with high (and often endemic) aquatic biodiversity [13]-regions that are now experiencing the most rapid rates of cropland expansion on earth [8,14].…”

River ecosystem metabolism and carbon biogeochemistry in a changing world

“…As budgets of river metabolism become more common 32,37,[50][51][52] , we can start to constrain global estimates of river GPP, ER and NEP. On the basis of the best available budget studies at present, we summarize the mean annual estimates of areal metabolic fluxes for tropical (GPP: 153 g C m −2 year −1 , ER: −760 g C m −2 year −1 , NEP: −606 g C m −2 year −1 ), temperate (GPP: 331 g C m −2 year −1 , ER: −591 g C m −2 year −1 , NEP: −260 g C m −2 year −1 ) and high-latitudinal (GPP: 279 g C m −2 year −1 , ER: −827 g C m −2 year −1 , NEP: −438 g C m −2 year −1 ) rivers (Supplementary Information; Table 1).…”