What are the effects of kelp forest decline on specific organisms in the food chain?

Kelp forests are complex and critical components of cold water rocky marine coastlines; their destruction--often as the result of overfishing--has a detrimental impact on the organisms that feed off of and around (or dwell in) these forests.

Kelp forests exist within the same ecosystems as fish, crabs, molluscs, sea urchins, marine mammals, algae, and many other biota. An important function of kept forests includes serving as a habitat and canopy for predatory fish. As kelp forests experience deforestation, the population of organisms that would typically serve as prey increases. This causes a significant shift in the food chain, with new predators emerging and normally flourishing populations dying off at unusual rates.

The rise of sea urchin populations also contribute to the deforestation of kelp through overgrazing. Urchins feed on kelp forests, and if left unchecked by apex predators who consume them (such as sea otters, whose deaths are also a result of overfishing), they can turn an entire kelp forest into an urchin barren.

To examine specific species, we'd have to narrow our subject to determine what kind of kelp forests you are interested in. For example, if we are examining the kelp forests of Southern California, we can return to the above example. Sea otters, spiny lobsters (Panulirus interuptus), and sheephead fish (Semicossyphus pulcher) act as the dominant carnivores in an ecosystem. These creatures feed on the dominant herbivores, such as the white sea urchin (Lytechinus anamesus), the purple sea urchin (Strongylocentrotus purpuratus), and the red sea urchin (S. franciscanus). If the dominant carnivores disappear (again, from overfishing or the presence of even larger predators, like killer whales), the above sea urchin populations will skyrocket; with more urchins feeding on the forest, the very sensitive kelp forests will be dramatically impacted. A paper published by The University of Maine called "Kelp Forest Ecosystems: Biodiversity, Stability, Resilience, and Future" beautifully explains this phenomenon. I've included the link below:

Whatever regulates sea urchin abundances or their grazing behaviour often controls the distribution and abundance of kelp forests... Predators are commonly strong interactors... and as such are the single most important agent controlling sea urchin populations... When sea urchin predators become the focus of intense and unsustainable fishing that extirpates them, hyperabundances of the sea urchins and kelp deforestation often result. 

This kind of event naturally has a detrimental impact on the other organisms feeding and living in this ecosystem, from molluscs and smaller fish to other algae and epibiota. 

For more information, please see the sources I've included below!